2 into 1's scavenge better than 2 into 2's.
Usually larger muffler volume to quell exhaust noise without affecting flow ( as compared to true duals.)

 

Here is a video I found showing how shitty drag pipes perform on Dyno tests and explaining the science behind it. They do Dyna tests on drag pipes and try to figure out how to improve their performances but the verdict is Drag pipes are junk and throw them into garbage...