I assume you are running your multimonitor setup either with native windows support, or in a 'span' configuration? Just create an image the size of both screens combined (e.g. for two 1024x768 screens side-by-side, create a 2048x768 image). Copy and paste one wallpaper on to the left of the image, and the other on the right. Save the file and set it as a tiled (yes, tiled) wallpaper. Enjoy.