I have a 9.2 Pioneer receiver now, replacing my previous 9.2 Onkyo receiver, and I suspect that the ".2" has only a marketing significance. That is, it's just like having a ".1" receiver with a Y-connector to connect another sub. However, the Onkyo did play a test signal through each sub separately during calibration, so it's possible that it somehow used information about the distance or volume of two subs independently. The Pioneer 9.2 receiver doesn't even do that, and in fact in its manual refers to the speaker system as "9.1", not "9.2" -- some engineer being more honest than marketing people, I suppose.
I like to use many subs of varying types and room positions. I have 3 subs in one room and 2 in another, currently. You see, more commonly, a recommendation for multiple subs of the exact same model and in nearly the same position in the room. In my opinion, the first arrangement tends to give the smoothest bass response, and the second gives the loudest bass response.
VP180, M80s, M22s, QS8(4), CSW S305s, EP500, Pioneer VSX-90
M2i, M3(2), Pio vsx-1020