I guarente someone has done it before, but you have to keep in mind 1) you will be adding more weight to your car without adding any performance and 2) it will cost more than its worth most likely unless you are building a show car (where dual exuasts are common and dont really help win points anymore).
Its a cool idea, im just trying to keep it in perspective for yah

. I have "dual exaust" (apexi dual N1) on my car and it didnt do much for power on the stock KA. On a custom set up you wont be able to test for the best flow and more likely than not there will be too many bends in the system for it to flow anybetter than a pre-fabricated single muffler cat back exaust (big companys got the big $$$ to put research into their products).