doable, but useless in terms of performance... a single large pipe will have overall less surface area that the exhaust gas comes in contact with the walls of the pipe compared to 2 medium sized pipes (dual 2.5" vs single 3.56" i believe)
Looking at the pix of the link domdogg posted, that still isn't a true dual exhaust. I true dual exhaust will have to totally seperate pipes dumping out exhaust individually.
Even though that is one beautiful setup, it's not dual exhaust. DeleriousZ is right. Don't think you'll see much in terms of performance. A setup like the one Domdogg displayed would be as close to perfect as you will get IMHO.
It could probably be done on a NA using the stock manifolds. Just make a nice dual engine pipe maintaining the separation at the flange.
Finding somewhere to put twin catcons could be tricky, else use a single catcon as a fakey X-pipe, but that would not be ideal. At least each of the twin catcons could be smaller since they individually only have to handle 1.5 litres of engine.
A well executed twin system could sound nice seeing how a 6 would have pulses spaced out further than a V8 at the same revs.
Exactly, what's the point. Gains would be minimal if at all. Domdogg is right, just put headers on the sucker and that's about it. Maybe even all new mandrel bent pipes with a high flow cat and muffler and that's it.
I wouldn't spend too much money on an exhaust for an N/A unless you absolutely wanted to for your own personal satisfaction and the knowledge that you have the absolute best stuff under the seats, then by all means do it. Like my grandaddy used to say, "everybody has to p!ss with his own d!ck!"