you should be the first to do a real dual exhaust.

get a 4-2-1 header and cut off the collector. leaving you with two pipes. get a couple of cats, i think a correct diameter for those pipes should be about 1.5". after the cats put an equalizer tube between the two pipes and run 'em back to your tl-s mufflers.

Use a 4-2-1 header with an O2 bung in the primaries (ala neuspeed N2, or even an OBX or something) and you could either use an O2 sensor emulator for the second O2 sensor, or extend some wires and put it in the equalizer pipe.