I don't know the science behind it (something to do with resonance pressure waves....) but a dual chamber pipe is better than a single chamber pipe (or muffler). I think they produce a wider range of pressure waves having two chambers, so the ideal backpressure is supplied to the engine at both ends of the throttle range. It may even be that one of the cones has more of an effect at low revs, whilst the other helps the engine when it's revving higher.

I think the number of outlets, or "stingers", is purely for looks as none of the high-end pipes have them. Mufflers and single chamber pipes (i believe you can get single chamber tuned pipes, although i've never seen one...) are used simply because they are cheaper than the 2-chamber tuned pipes and mufflers are much more compact.

EDIT - This is just based on what i've seen. I could be (and probably am) completely wrong.