the nvidia control panel has the ability to put seperate wallpapers on each monitor.
Or you could make a single wallpaper comprised of your two favorite papers in an image that's 2560x1024 (assuming the resolution of each monitor is 1280x1024 to begin with.) Whatever your resolution is, just double the width and that'd work.
Like I said though, right clicking on desktop and getting to your nvidia properties I know it's in there somewhere to actually assign each monitor a seperate image but it's been a while since I've done that.