This seems to be an issue with qt-copy.
When I build my simple application with qt-copy, it reports a single screen. On the other hand, building against a stock Qt 4.4 results in two screens reported. 

Same here on FreeBSD. I have setup a dual-head system using Nvidia's Twinview, and on startup, both screens do come on, but Plasma was only loaded in the left one. The right screen I could move my mouse to, or drag open windows over to, but Plasma was *not* running on it. 

I found a kludge to force it to work though. I created a new panel on my left screen, and dragged it over to the right screen. Then I re-logged in, and plasma started properly on the right screen now. 

Additional, if I remove my .kde4/share/config/plasma-appletsrc and login, then it detects the 2nd screen. However, if I already have this file configured for 1 screen, and then attempt to add a 2nd, it never auto-detects the 2nd monitor. 

Created attachment 27505 [details]
xorg.conf file

Whoops. Here's the text regarding my attachment. 

I have the same issue as well.

OS: FreeBSD 7.1-PRERELEASE
nvidia drivers: current from ports; nvidia agp and no linux compat

I first setup the system using twinview and although "Desktop" control panel showed the full combined resolution the 2nd desktop wouldn't display. I could drag items over to it, but they would leave fragments of the desktop and window streaks.

I switched my xorg.conf to be 2 independent monitors not using twinview. Now, I get nothing and I can't even drag windows over to the second. However, if I wait for the screen saver to kick on it will display on both screens. kxrandr control shows 2 screens as an option, but otherwise KDE doesn't seem to think their are 2 monitors. The "Desktop" control panel now thinks there is just one monitor of 1680x1050.

Attached is my xorg.conf

can you please check again with a more recent trunk (or soon beta1)? the inclusion of kephal should have made things work more nicely toghether. =)

/me seconds the question from comment #6 ... would be great to know if that problem is still there and/or what other probs do exist now. Thanks in advance for any feedback :)

Yup, the problem is still there. Using svn 907177, this is what happens:

My setup:
-------------------------------
|                |            |
|   1680x1050    | 1280x1024  |
|                |            |
|                |-------------
------------------

1. Login with a single display:
$ xrandr
Screen 0: minimum 512 x 384, current 1680 x 1050, maximum 1680 x 1050
default connected 1680x1050+0+0 0mm x 0mm
   1680x1050      50.0*
   1600x1024      51.0
   [etc...]
   512x384        65.0

After using nvidia-settings to enable the second display:
$ xrandr
Screen 0: minimum 512 x 384, current 2960 x 1050, maximum 2960 x 1050
default connected 2960x1050+0+0 0mm x 0mm
   1680x1050      50.0     66.0
   1600x1024      51.0
   [etc...]
   512x384        65.0
   2960x1050      66.0*

There is no root window / no plasma on the second display. You can drag windows onto it, but Xinerama hints are ignored: trying to maximise a window from the second display gets it maximised on the first.

Restarting kwin / plasma doesn't help, and neither does further switching of modes.

2. Login with both displays:
$ xrandr
Screen 0: minimum 2960 x 1050, current 2960 x 1050, maximum 2960 x 1050
default connected 2960x1050+0+0 0mm x 0mm
   2960x1050      66.0*

Everything works just fine, there is an additional plasma button on the second screen, I can add widgets / panels, maximise windows etc.

The rather interesting part comes after disabling the second monitor.
$ xrandr
Screen 0: minimum 1680 x 1050, current 1680 x 1050, maximum 2960 x 1050
default connected 1680x1050+0+0 0mm x 0mm
   2960x1050      66.0     50.0
   1680x1050      50.0*

Everything works fine, until trying to enter systemsettings -> display. At this point compositing crashes, and X starts eating 100% CPU. After managing to run an xrandr -s 0, CPU usage went down, but no compositing.
The "Desktop effects" checkbox in systemsettings was still checked. Uncheck/re-check/apply results in "Failed to activate desktop effects using the given configuration options. [...] Check your X configuration. " etc.

However, compiz --replace works fine, and doing kwin --replace afterwards allows compositing to be enabled again.

*** This bug has been confirmed by popular vote. ***

On current trunk (r961118) When I enable the second monitor (using nvidia settings, for example) the secondary monitor is enabled but desktop effects are disabled, I've to restart kwin for re-enable desktop effects.

Qt 4.5.0
Xorg 1.6.1
Nvidia 180.44

This bug might be related to bug #156475

Please not that the Debian bug 529487 [0] was forwarded to this bug report.

[0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=529487

This bug report have soon 1 year birthday. Is that a good thing?

*** This bug has been marked as a duplicate of bug 156475 ***