There are some factors to consider when using multiple or single subs. In general, two identicals placed in diferent points in a room, but not opposite each other, can generate a more uniform,if not more powerful, bass dispersion. Also, if you set the crossover point above 100 hz, the bass frequencies begin to be directional so that a dedicated sub for each channel is recommended to get stereo separation down to the lowest possible notes.
While it is true that the smaller drivers can give better performance as you go up the frequency ladder, I fail to see the advantage of stacking two different subs so that one sub can deliver above 80hz and the other below. If you do this, there is more likelihood the smaller sub will be impinging on the main speakers so that you may get peaks or cancellations in the 80hz to 100 hz where both dirvers are operating. Unless of course you set the crossover high enough, in which case, you get directionality problems that must be addressed by using 2 subs, one for each channel. At the same time, the smaller sub will offer the same interaction with the larger sub to smear the overall LF delivery. Just my thoughts.
