A related question to the turbocharger discussion -
So if you're designing for a 4" diameter pipe, would two 2.8" diameter pipes provide the same effect? The area of two 2.8" pipes is roughly the same as that of a 4" pipe, if you don't feel like doing the math.
Two 2.8" pipes will not flow as well as a single 4" pipe, there's about 40% more surface area inside the 2x2.8" pipes, which means more heat loss, more friction in the exhaust flow, and 40%+ more weight.