As you can tell from my signature I've created a fair share of desktop backgrounds and many are for dual monitors
What TSL said, find a picture that's 2x or larger than the sum of your monitors' resolutions (that's ideal, so it looks nice and sharp) and crop and trim it to size. I actually use a Photoshop action for this, fully automated process
Or, PM me if you find such picture and I'll crop and size it for you.