hello!
i also have two monitors of that size - the total desktop size is 3360x1050.
that said, you can find several great desktops of that size here:
http://widewall.blogspot.com/http://www.3360x1050.com/but know that for a dual-monitor setup, osx processes each monitor's wall separately, so you'll have to take them into photoshop and split them into two 1680x1050 files.
alternatively, mandolux (
http://www.mandolux.com/ ) has tons of beautiful walls, split into multiple 1680x1050 files, so you can put them together without any sort of extra work.
to configure any of these (1680x1050 files for each monitor, remember), drop the images into the "Pictures" folder under your user's house thing. then, go to system preferences, desktop & screen saver, click to the desktop tab, then the 'pictures folder' on the left listing for each monitor, and select what you want.
ALSO
what i've been compelled to do instead of this, and would advise you to try it if you've got the processing power - look into a dashboard widget called 'wallsaver'. you can configure any screensaver to work as your background, and the "Arabesque" one is absolutely stunning on a dual monitor setup.
CHEERS / ENJOY!