I tried searching for a solution, but couldn't find one... so maybe I'll try my luck here.
I want to use the dual screen mosaic for my wallpaper, but the problem is my monitors have different resolutions. So my main monitor's desktop is nice, and you can see everyone on the horde, but the alliance on the other monitor are cut off.
You could just edit separate desktops out of the original dual screen image file. Take the big file, edit a horde side for your left desktop, then edit a smaller desktop image for the alliance side.