Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
There is no wallpaper spanning, since each screen is independent. What you would need to do is to create two desktop pictures (wallpaper), one for your left and one for your right, (or however it is oriented), then select the correct one for each screen.

TEG
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.