oh, this is also my 2cents...

system 1: (twin mufflers)
each muffler is say 80db, and flows 300CFM
thats a total of 600CFM flow...

system 2: (single muffler)
sound level of 90db, flows 500CFM...
total of 500CFM flow...

System two is quieter, and flows more...

(these numbers are made up, i have no idea what an exhaust flows at, im just trying to show the difference)