deleting xorg.conf and letting it use xinerama mode works around the problem. 
Multihead is still broke. 

xorg.conf:

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 1.0  (buildmeister@builder63)  Tue Oct 20 21:00:15 PDT 2009

# File generated by xorgconfig.
#
# Copyright 2004 The X.Org Foundation
#
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
# The X.Org Foundation BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF
# OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
# 
# Except as contained in this notice, the name of The X.Org Foundation shall
# not be used in advertising or otherwise to promote the sale, use or other
# dealings in this Software without prior written authorization from
# The X.Org Foundation.
#
# **********************************************************************
# Refer to the xorg.conf(5x) man page for details about the format of 
# this file.
# **********************************************************************
# **********************************************************************
# Module section -- this  section  is used to specify
# which dynamically loadable modules to load.
# **********************************************************************
#
# **********************************************************************
# Files section.  This allows default font and rgb paths to be set
# **********************************************************************
# **********************************************************************
# Server flags section.
# **********************************************************************
# **********************************************************************
# Input devices
# **********************************************************************
# **********************************************************************
# Core keyboard's InputDevice section
# **********************************************************************
#Section "InputDevice"
#    Identifier	"Keyboard1"
#    Driver	"kbd"
# For most OSs the protocol can be omitted (it defaults to "Standard").
# When using XQUEUE (only for SVR3 and SVR4, but not Solaris),
# uncomment the following line.
#    Option     "Protocol"      "Xqueue"
#    Option "AutoRepeat" "500 30"
# Specify which keyboard LEDs can be user-controlled (eg, with xset(1))
#    Option	"Xleds"      "1 2 3"
#    Option "LeftAlt"     "Meta"
#    Option "RightAlt"    "ModeShift"
# To customise the XKB settings to suit your keyboard, modify the
# lines below (which are the defaults).  For example, for a non-U.S.
# keyboard, you will probably want to use:
#    Option "XkbModel"    "pc105"
# If you have a US Microsoft Natural keyboard, you can use:
#    Option "XkbModel"    "microsoft"
#
# Then to change the language, change the Layout setting.
# For example, a german layout can be obtained with:
#    Option "XkbLayout"   "de"
# or:
#    Option "XkbLayout"   "de"
#    Option "XkbVariant"  "nodeadkeys"
#
# If you'd like to switch the positions of your capslock and
# control keys, use:
#    Option "XkbOptions"  "ctrl:swapcaps"
# These are the default XKB settings for Xorg
#    Option "XkbModel"    "pc105"
#    Option "XkbLayout"   "us"
#    Option "XkbVariant"  ""
#    Option "XkbOptions"  ""
#    Option "XkbDisable"
#    Option "XkbModel"	"pc104"
#    Option "XkbLayout"	"us"
#EndSection
# **********************************************************************
# Core Pointer's InputDevice section
# **********************************************************************
#Section "InputDevice"
## Identifier and driver
#    Identifier	"Mouse1"
#    Driver	"mouse"
#    Option "Protocol"    "Auto"
#    Option "Device"      "/dev/input/mouse0"
#    Option "Buttons" "5"
#:wq
#Option "ZAxisMapping" "4 5"
# Mouse-speed setting for PS/2 mouse.
#    Option "Resolution"	"256"
# When using XQUEUE, comment out the above two lines, and uncomment
# the following line.
#    Option "Protocol"	"Xqueue"
# Baudrate and SampleRate are only for some Logitech mice. In
# almost every case these lines should be omitted.
#    Option "BaudRate"	"9600"
#    Option "SampleRate"	"150"
# Emulate3Buttons is an option for 2-button Microsoft mice
# Emulate3Timeout is the timeout in milliseconds (default is 50ms)
#    Option "Emulate3Buttons"
#    Option "Emulate3Timeout"    "50"
# ChordMiddle is an option for some 3-button Logitech mice
#    Option "ChordMiddle"
#EndSection
# **********************************************************************
# Other input device sections 
# this is optional and is required only if you
# are using extended input devices.  This is for example only.  Refer
# to the xorg.conf man page for a description of the options.
# **********************************************************************
#
# Section "InputDevice" 
#    Identifier  "Mouse2"
#    Driver      "mouse"
#    Option      "Protocol"      "MouseMan"
#    Option      "Device"        "/dev/mouse2"
# EndSection
#
# Section "InputDevice"
#    Identifier "spaceball"
#    Driver     "magellan"
#    Option     "Device"        "/dev/cua0"
# EndSection
#
# Section "InputDevice"
#    Identifier "spaceball2"
#    Driver     "spaceorb"
#    Option     "Device"        "/dev/cua0"
# EndSection
#
# Section "InputDevice"
#    Identifier "touchscreen0"
#    Driver     "microtouch"
#    Option     "Device"        "/dev/ttyS0"
#    Option     "MinX"          "1412"
#    Option     "MaxX"          "15184"
#    Option     "MinY"          "15372"
#    Option     "MaxY"          "1230"
#    Option     "ScreenNumber"  "0"
#    Option     "ReportingMode" "Scaled"
#    Option     "ButtonNumber"  "1"
#    Option     "SendCoreEvents"
# EndSection
#
# Section "InputDevice"
#    Identifier "touchscreen1"
#    Driver     "elo2300"
#    Option     "Device"        "/dev/ttyS0"
#    Option     "MinX"          "231"
#    Option     "MaxX"          "3868"
#    Option     "MinY"          "3858"
#    Option     "MaxY"          "272"
#    Option     "ScreenNumber"  "0"
#    Option     "ReportingMode" "Scaled"
#    Option     "ButtonThreshold"       "17"
#    Option     "ButtonNumber"  "1"
#    Option     "SendCoreEvents"
# EndSection
# **********************************************************************
# Monitor section
# **********************************************************************
# Any number of monitor sections may be present
# **********************************************************************
# Graphics device section
# **********************************************************************
# Any number of graphics device sections may be present
# Standard VGA Device:
#Section "Device"
#    Identifier	"Standard VGA"
#    VendorName	"Unknown"
#    BoardName	"Unknown"
# The chipset line is optional in most cases.  It can be used to override
# the driver's chipset detection, and should not normally be specified.
#    Chipset	"generic"
# The Driver line must be present.  When using run-time loadable driver
# modules, this line instructs the server to load the specified driver
# module.  Even when not using loadable driver modules, this line
# indicates which driver should interpret the information in this section.
#    Driver     "vga"
# The BusID line is used to specify which of possibly multiple devices
# this section is intended for.  When this line isn't present, a device
# section can only match up with the primary video device.  For PCI
# devices a line like the following could be used.  This line should not
# normally be included unless there is more than one video device
# intalled.
#    BusID      "PCI:0:10:0"
#    VideoRam	256
#    Clocks	25.2 28.3
#EndSection
# Device configured by xorgconfig:
# **********************************************************************
# Screen sections
# **********************************************************************
# Any number of screen sections may be present.  Each describes
# the configuration of a single screen.  A single specific screen section
# may be specified from the X server command line with the "-screen"
# option.
# **********************************************************************
# ServerLayout sections.
# **********************************************************************
# Any number of ServerLayout sections may be present.  Each describes
# the way multiple screens are organised.  A specific ServerLayout
# section may be specified from the X server command line with the
# "-layout" option.  In the absence of this, the first section is used.
# When now ServerLayout section is present, the first Screen section
# is used alone.
# Section "DRI"
#    Mode 0666
# EndSection

Section "ServerLayout"

# The Identifier line must be present
# Each Screen line specifies a Screen section name, and optionally
# the relative position of other screens.  The four names after
# primary screen name are the screens to the top, bottom, left and right
# of the primary screen.  In this example, screen 2 is located to the
# right of screen 1.
#	Bizarre that I had to reverse these when I migrated to xorg 1.5.3
#	And then I had to switch them back!
#	Screen "Screen2"
#	Screen "Screen1" Rightof "Screen2"
# Each InputDevice line specifies an InputDevice section name and
# optionally some options to specify the way the device is to be
# used.  Those options include "CorePointer", "CoreKeyboard" and
# "SendCoreEvents".
#    InputDevice "Mouse1" "CorePointer"
#    InputDevice "Keyboard1" "CoreKeyboard"
    Identifier     "Simple Layout"
    Screen         "Screen1" 0 0
    Screen         "Screen2" RightOf "Screen1"
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
EndSection

Section "Files"

# The location of the RGB database.  Note, this is the name of the
# file minus the extension (like ".txt" or ".db").  There is normally
# no need to change the default.
# Multiple FontPath entries are allowed (which are concatenated together),
# as well as specifying multiple comma-separated entries in one FontPath
# command (or a combination of both methods)
# 
# 
#    FontPath   "/usr/share/fonts/CID/"
#    FontPath   "/usr/share/fonts/Speedo/"
#    FontPath   "/usr/share/fonts/TrueType/"
#    FontPath   "/usr/share/fonts/freefont/"
# The module search path.  The default path is shown here.
    FontPath        "/usr/share/fonts/misc/"
    FontPath        "/usr/share/fonts/TTF/"
    FontPath        "/usr/share/fonts/Type1/"
    FontPath        "/usr/share/fonts/75dpi/"
    FontPath        "/usr/share/fonts/100dpi/"
    FontPath        "/usr/share/fonts/local/"
EndSection

Section "Module"

# This loads the DBE extension module.
#    Load        "dbe"  	# Double buffer extension
# This loads the miscellaneous extensions module, and disables
# initialisation of the XFree86-DGA extension within that module.
#    SubSection  "extmod"
#      Option    "omit xfree86-dga"   # don't initialise the DGA extension
#    EndSubSection
# This loads the font modules
#    Load        "type1"
#    Load        "speedo"
#    Load        "freetype"
#    Load        "xtt"
# This loads the GLX module
#    Load       "glx"
# This loads the DRI module
    # Load       "dri"
    Load           "glx"
EndSection

Section "ServerFlags"

# Uncomment this to cause a core dump at the spot where a signal is 
# received.  This may leave the console in an unusable state, but may
# provide a better stack trace in the core dump to aid in debugging
#    Option "NoTrapSignals"
# Uncomment this to disable the <Crtl><Alt><Fn> VT switch sequence
# (where n is 1 through 12).  This allows clients to receive these key
# events.
# This allows clients to receive this key event.
# sequences.  This allows clients to receive these key events.
#    Option "Dont Zoom"
# Uncomment this to disable tuning with the xvidtune client. With
# it the client can still run and fetch card and monitor attributes,
# but it will not be allowed to change them. If it tries it will
# receive a protocol error.
#    Option "DisableVidModeExtension"
# Uncomment this to enable the use of a non-local xvidtune client. 
#    Option "AllowNonLocalXvidtune"
# Uncomment this to disable dynamically modifying the input device
# (mouse and keyboard) settings. 
#    Option "DisableModInDev"
# Uncomment this to enable the use of a non-local client to
# change the keyboard or mouse settings (currently only xset).
#    Option "AllowNonLocalModInDev"
#     Option "Xinerama" "True"
#     Option "AllowEmptyInput"		"false"
    Option         "DontVTSwitch" "False"
# Uncomment this to disable the <Crtl><Alt><BS> server abort sequence
    Option         "DontZap" "False"
# Uncomment this to disable the <Crtl><Alt><KP_+>/<KP_-> mode switching
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "keyboard"
EndSection

Section "InputDevice"
    # generated from data in "/etc/conf.d/gpm"
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol"
    Option         "Device" "/dev/input/mice"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "Monitor"

# HorizSync is in kHz unless units are specified.
# HorizSync may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY.  REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.
#    HorizSync	30-64         # multisync
#    HorizSync	31.5, 35.2    # multiple fixed sync frequencies
#    HorizSync	15-25, 30-50  # multiple ranges of sync frequencies
# VertRefresh is in Hz unless units are specified.
# VertRefresh may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY.  REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.
    Identifier     "Viewsonic-VG910s"
    HorizSync       30.0 - 82.0
    VertRefresh     50.0 - 75.0
EndSection

Section "Monitor"
    Identifier     "Viewsonic-VG910s1"
    HorizSync       30.0 - 82.0
    VertRefresh     50.0 - 75.0
EndSection

Section "Device"
    Identifier     "msi-nx6800-ultra"
    Driver         "nvidia"
    BusID          "PCI:1:0:0"
    Screen          0
EndSection

Section "Device"
    Identifier     "msi-nx6800-ultra1"
    Driver         "nvidia"
    BusID          "PCI:1:0:0"
    Screen          1
EndSection

Section "Screen"

#    Subsection "Display"
#        Depth       8
#        Modes       "1280x1024" "1024x768" "800x600" "640x480"
#        ViewPort    0 0
#    EndSubsection
#    Subsection "Display"
#        Depth       16
#        Modes       "1280x1024" "1024x768" "800x600" "640x480"
#        ViewPort    0 0
#    EndSubsection
    Identifier     "Screen1"
    Device         "msi-nx6800-ultra"
    Monitor        "Viewsonic-VG910s"
    DefaultDepth    24
    SubSection     "Display"
        Viewport    0 0
        Depth       24
        Modes      "1280x1024"
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen2"
    Device         "msi-nx6800-ultra1"
    Monitor        "Viewsonic-VG910s1"
    DefaultDepth    24
    SubSection     "Display"
        Viewport    0 0
        Depth       24
        Modes      "1280x1024"
    EndSubSection
EndSection

If 'xrandr' does not show the second screen, there is nothing Plasma developers can do, as the configuration that is reported by xrandr is used by Plasma.

Please ask at nvidia forums how to solve this issue.

Thanks for the report Jonathan. 

I've tested this on my current setup but cannot reproduce.

Can you please confirm if this issue still persists for you or if this bug report can now be resolved, thanks.

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least
15 days. Please provide the requested information as soon as
possible and set the bug status as REPORTED. Due to regular bug
tracker maintenance, if the bug is still in NEEDSINFO status with
no change in 30 days the bug will be closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please
mark the bug as REPORTED so that the KDE team knows that the bug is
ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

This bug has been in NEEDSINFO status with no change for at least
30 days. The bug is now closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

Thank you for helping us make KDE software even better for everyone!