Just a few random thoughts
One 3" pipe flows the same as 2 - 2" pipes if calulated as you would for pressureized liquid flow. I think the advantage of one three inch pipes is less surface area to cool the exhaust gases and to cause friction losses along the pipe wall. If you go to 2 - 3" pipes I think your flow loss due to lack of velocity would hurt your low end performance at least on an unblown engine. The effects on a turbo charged engine might be less.
The effects of 1 5/8, 1 3/4, and 2" pipes on small block chevys was studied years ago by the larger header companies and they discovered that the operating RPM range made a major difference in what was required.
This is why exhaust extractors for high performance engines are normally tuned for a given rpm range usually narrow.
Just for fun figure how many cfm a 250 turning at 1000rpm puts out, then figure the cfm of the same engine turning at 7000 rpm and see if you think the pipe diameter requirements would be different.
