you definitely need to checkout
mandolux. hands down the best multi monitor wallpaper out there. Why on earth don't more people know about an artist that good? You can buy high res versions of his work over at istockpro too.
Mandolux is great, I definitely agree there. There are some of Mando's dual monitor backgrounds at
http://www.dualmonitorbackgrounds.com too
The advantage of using dmb is that it's categorized, so instead of going through one image at a time in a linear fashion (as is the case on Mandolux) you can find what you're looking for much faster on dmb, whether it's Mando's art or any of the hundreds of other people's art.
I absolutely think people should check out mandolux.com though, it's a nice site for looking at amazing artwork, I just think dmb is more useful and intuitive for the purpose of finding dual monitor backgrounds.
Mandolux is more of a portfolio/blog site, showcasing amazing backgrounds in a linear, chronological manner, which I don't think is ideal for people looking for backgrounds, whereas dmb allows people to go on, see the categories they want, and view thumbnails before they click through to the full sized image.
There's also a search function, image rating and comments.
Also on Mandolux you never know whether you will get a background for 2 monitors or 3 or 4 or 6, it's all the luck of the draw.
Know what I mean?