I can see positives for both having the same subs and for having different ones. I would believe all subs would have slightly different response curves so you could use that to potentially create a smoother overall response, but it could also potentially create larger peaks and valleys depending on how the compliment each other.
IMO without spending a ton of time analyzing it I'd just say either way is fine.
My personal experiences with multiple subs has left me weary of cancellation effects. I assume it has to do with how the walls reflect the sound into the waves of the other sub. That said I'm planning on used two subs in my new HT.
