if i right click on my desktop and select 'destkop settings' and then select a new wallpaper, it is only changed on the physical screen, not on the entire destkop (which is composed of two physical screens linked via xinerama). is that the way it is defined to work? if so, why?

/mark

It is defined to work in this way because it leads to less corner cases and other such bugs in Plasma. I recommend simply setting two seperate images as your Desktop Background.

It also allows for additional features (different types of desktops on the two screens, for instance), works better when you plug in and unplug screens, and works better for different-resolution screens.