Having a dual monitor setup, I love finding good desktop pictures (wallpapers) made for dual displays. Tonight I came across some great ones over at Studio Twenty Eight, including this set called, Summer. The site’s author, Francisco Javier Ocasio Gotay, also made Spring, Autumn, and Winter, for this same scene, and provides single monitor versions. Thanks Francisco!
If you don’ like these, he has several others to choose from. Check them out.
Studio28 is great, he makes some good desktop themes too (to use with StyleXP).
There are a whole lot of dual monitor backgrounds at http://www.dualmonitorbackgrounds.com too, hope this helps in your search for more :)