You'll need some image editing software, such as Photoshop or Gimp or even Preview will work in a pinch. You need to crop the image to the size of each screen and then save it as two different images. For instance, I have two 1050x1680 monitors, so each image needs to be 1050x1650. Then, you just load them up as you would any other wallpaper in System Preferences.
STL, huh? Where'd you go to high school? (;