cgm: sorry for the late reply, been really busy!
There have been several builders who did a double kegger using only one fridge. In fact, during a trip to a local bar owned by a friend of mine, I was surprised to see the small compressor they use for several large keg and beer coolers. They trick to all this is held in one word: CIRCULATION.
Add several fans to be sure air is moving inside the box. If you have frost on the freezer plate, you should be able to cool the entire box to nearly freezing by circulating air across the plate area and around the keg box.
Hope that helps!
Steve