I think there are a few other guys who've done the twin turbo setup, two of them never completed. If I were doing it, I'd use the already present manifolds and exhaust them into two 2" pipes. You could run two metal substrate cats (they're "thin" and would fit staggered in the factory location no problem) and then into a muffler setup. For mufflers, you can safely go into a collector of say 3" and not lose any performance, as the exhaust gases have cool enough.
Alternatively, you could go with two separate mufflers, a true twin exhaust, even going so far to modify the valance to exit one on each side. That would look swell.
Making a collector isn't hard if you have a bandsaw or are really careful with a hacksaw. You essentially cut two pipes on angles, then once they're positioned together, find the point on the merged area in which the two make a smooth oval and cut the whole thing off flush. Then get larger pipe, hammer it down to make the oval and weld that on.
The angles, oval size and diameter of the larger pipe are the variables in the equation. I know there is some hot math to figure it out (or use Solidworks if you're smart, i'm not) or trial and error. Pipe is cheap, I'd go with the latter.