Hi Grank It Up,
You could combine a sub with a 12-inch driver with a 10-inch-driver sub--it depends how much bass extension and output you want. If anything, the different characterisitics of the two subs (they won't be identical in the rate of bass rolloff, or distortion, or deep bass extension) might be complementary to each other.
But the real virtue of two subs, as Ian pointed out, is the ability to smooth out bass response over a broader listening area than would otherwise be possible with one sub. Standing waves with nodes of bass cancellation and bass emphasis are an inevitable product of any room (a room with square dimensions is the very worst to generate standing waves), and using two subs is an effective remedy.
Regards,
