A cat-back is just an exhaust system that leads into the cat, a dual system never joins together. But I think you're referring to a Y-pipe vs a X-pipe vs a H-pipe. A Y-pipe has both headers leading into a single cat-converter and then leading out through a single muffler. This system relieves back pressure in the mufflers, but has stock limitations in the cat. An X-pipe has each header leading into a separate cat-converter then the pipes join together to equalize pressure, before separating again into two separate mufflers (most effective system). Or the H-pipe (loudest) has two separate pipes coming off each header with it's own cat and muffler.
If you want to increase sound go with the H-pipe and cut out the mufflers to make a straight pipe.
