Functional twin outlet? That would require a complete re-design of the exhaust system.
a) Complete twin system...one from each head would require a recalculation of the diameter of pipe used, length of downpipe, length between elbow and centre box/bullet, etc to achieve the harmonics and pulse characteristics required. Frankly, a lot of calculations and not worth the time, cos you might actually lose low end power because there is just not enough air-flow through each bank of cylinders. An estimate would be to use no larger than 1.25" piping and keep both lengths exactly the same length.
b_ X-pipe....like that on the 350Z...same effect as above though I would really like to try it out some day... :)
Gotta think a little more and do more research, but popular opinion is that 3L is just too little cubic capacity for a twin out unless...you have a turbo...