I did the same thing in the Plus line except I started with a 20-39PC+ and added a PB12-Plus/2 after a few weeks.
You may not "need" it, but if you "want" it why not? One thing is for sure, you can get a lot of bass and headroom, and not work your subs nearly as hard, as trying to do the same with one (even if it a /2). Mine are not colocated and I still get ~ 4db - 5db more with two. Colocated you should be able to get 6db (or about 2X as you might expect.)
One thing two subs can do in a negative sense is cause even more bizzare behavior with regard to room modes and as such require careful phase alignment ("easier" with SVS's continuous phase control). Have you measured your FR in the room yet and are you using the EQ in the sub or an external EQ? Getting that flattened out a bit will also give you the perception of "more" bass.
We're all bassaholics - some just choose to feed their habit more than others...
Doug