Gentlemen:
I believe Ron hit the nail on the head. I am sure I have seen a Flowmaster with two inlets and two outlets, so they have done that already. I personally had an old flowmaster from a vehicle, cut it apart, and fabricated a two in/two out muffler based on the basic design inside the old unit out of stainless sheet(.080) and it sounds fine, a bit on the loud side but that is what we were after anyway. Any good fab shop should be able to knock one of these together for a decent price, and you can put bracketry on the unit for a mount, heat shield whatever, maybe I should look and see if I have my old sketches and knock out a few.!
Cheers
Phil