You have a dual monitor setup and you want to set for each one a special background, no problem now comes ImageMagick into play to montage two different backgrounds into a single one. Get to work:
aptitude install imagemagick
Montage backgrounds one and two(1.jpg and 2.jpg) into a large one (background.jpg).
montage 01.jpg 02.jpg -tile 2x1 -geometry "2560x1600+0+0" -mode concatenate background.jpg