Make a new, blank, large image (e.g. 2048x768 if you run a 1024x768 resolution), but large enough so that it will fit exactly on both monitors, and put 2 different wallpapers each occupying half of the space. Then use it as your wallpaper (by going into Display Properties), and set the position to "Tile."
PowerToys has a desktop manager allowing virtual desktops, as with ATI's Hydravision software (if you own an ATI Radeon-based card), but if you don't have access to such, I prefer the old-fashioned method above.
everyone with more then one monitor needs to check out ultramon at http://www.realtimesoft.com/ultramon/ you will swear by it after using it once I have three monitors and each has its own res. and background. you can even have each monitor run a different screen saver!