Depending on the specifics of the config used, I have observed the following results in KDEPlasma 5.15
- Screen0 shows a black screen on the primary output display, desktop wallpaper on secondary display, Screen1 shows a full desktop+panel. All applications and menus from Screen1's desktop start on Screen0's primary display, and cannot be moved between screen0 displays.
- Screen0 shows a black screen on the primary output display, desktop wallpaper on secondary display, Screen1 shows a full desktop+panel. All applications and menus from Screen1's desktop start on Screen0's secondary display, and cannot be moved between screen0 displays.
- Everything is on Screen1. Screen0 is accessible, but nothing can be opened on it, nothing opens to it, and nothing but the mouse cursor can be seen on it.
And a few other very similarly broken results.
As an aside, when using XFCE, Screen1 is blank, Screen0 works correctly.
When using Xinerama, Screen1 was, the one time I tried it, a black background, and screen0 had everything scaled and placed incorrectly, including having main panels completely off screen and inaccessible, as well as several windows.
In all cases, attempting to send anything to screen1 fails, reporting that screen1 does not exist, and using xrander only shows displays associated with screen0. These displays do show up with xrandr when screen1 is disabled, but obviously, they have no output.