On our cars not much. Our 2 manifolds merge to one and then flow all the way to just after the rear axle as one pipe.
If you wanted a dual exhaust (2 mufflers) you'd make another Y (reversed this time) and convert the 1 pipe to 2. That the the factory set-up on the V6's.
If you wanted a single, you'd just keep that single pipe all the way back to the bumper, and mount 1 muffler on it.
Hope that helps.