Another question one might ask is with such an arrangement with different subs capable of significantly different bass extension capabilities, is where do you set the volume levels? Obviously, regardless of size, they have to be balanced. You may run in to a problem with the small subs on some movies that have very loud and low bass extension that they just can't handle and overdrive them while the bigger subs handle this same bass quite easily.
What I meant, but didn't say what that the little subs would have a low pass filter of say 40Hz and a high pass filter of 80Hz. The sub with a large driver would have a high pass filter of 40Hz.