Colocating bass drivers is a bit of an inexact science, though... and while you may get overlapping waves doubling expected sound pressures, others may not... at best you'd end up with "4 subs' worth of bass"... at worst you'd end up with a non-linear response throughout the handled frequencies.
And for a bunch of people that pee and moan about speaker response graphs that don't lie flat enough to check a ruler by, intentionally sacrificing linear response for sound pressure seems odd.
But what do I know...? People buy clocks to syncopate their audio systems then play compressed audio files through them.
Bren R.