Feel free to shoot this down, this is just me thinking out loud kinda...
A 10 (generally) has better "kick" cos it has a smaller (therfore) lighter cone than a 12, so fed with the same power will be able to move more "accurately". (e.g. the amp will have more control over the smaller weight being moved)
A 12 gets lower becasue with a given frequency the subs fed with the same power will have similair excuriosn, but the 12 wil have more surface area, and therefore be louder (more effecient) and therfore reproduce the hard to hear lower frequencies better.
I can't htink of a better explanation IF everything else about the sub is the same?!
So therefore, 2 10s of same quality (NOT half the price of th 12) should in theory go AS low (cos they will have same or greater surface area) and kick better than 1 12. Assuming u can power every sub to its full capabilities.
(All this isn't taking into account "finances" of powering the two 10s vs 1 12, and the price of two 10s vs 1 12.)
What people think?!?!?!

(am i crazy?)