Bug 435131

SUMMARY
When using 2 screens, the SDDM login shows in both screens INDEPENDENTLY! For example, you can type a password in one screen and a different password on the other, or select 2 different users on each screen.

STEPS TO REPRODUCE
1. Boot Kubuntu. Make sure you have a second screen connected.
2; You're at SDDM. Type a password, press left/right,etc. 

OBSERVED RESULT
Notice how the other screen doesn't change. You can even type a different password on the other screen, or select a different user. What will happen when you press enter? This is not right.

EXPECTED RESULT
If having both screens is allowed, they should be in sync. Anything you do in one must be reflected on the other.
Or, you could show the login prompt in just one of the screens, and just empty BG on the other (though this is likely a bad idea!).

SOFTWARE/OS VERSIONS
OS: Kubuntu 20.04 64-bit
Linux Kernel: 5.4.0-70-generic
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
Likely irrelevant, but I'm on a laptop, connecting to a TV/monitor, using a normal 15-pin VGA cable, not HDMI.