The best way I can think to do it is to buy another muffler and use some wire or something to hold it in place on the opposite side of the functional can.

FYI: Backpressure is always a bad thing. Air velocity (or, high air velocity) is a good thing. The trick is getting the lowest backpressure with the highest velocity. Too big a pipe, and velocity is lost. Too small, backpressure is too high. True dual exhaust falls under the catagory of "too big."