Where is the science? The whole purpose of an expansion chamber is to tune the pressure waves going out of the cylinder. That is why there are high, mid, and low range pipes. They are tuned for different RPM ranges. A "2 into 1" pipe would have two sets of waves conflicting with each other. They have no real "hit", but a nice even pull through the RPM's.
Also, in my opinion, they are the best sounding pipe out there. When you here one in the woods it sounds like an indy car rippin' up the hills.