You need to increase the size by at least 18″. That is the base diameter of a metal (aluminum) keg. The rubber coated kegs are about 1 inch wider.
I have not tried it myself, but if you place fans properly, and do a good insulation job, it should work for 2 kegs.
I have looked at many old bars in my area (Wisconsin, we have a bar on every corner) and most older beer & keg boxes use a single compressor to cool several boxes. The trick is to move as much air as possible.
See the pinned topics in the Kegger Bar section for more info.