As Shady said, if you absolutely need maximum output per Sub, get large "Dualies." Each co-located Sub will increase SPL Output... Is it +6dB, or +3, I don't recall right now.
The flipside is that you will still want Subs in multiple locations as that will help smooth out the bass response in-room. Co-located Subs will not do this as they behave as a single unit.
(Colocated meaning two drivers located withing ~3' of each other, so a Dual Sub like you posted above would be the equivalent of stacking 2 Monolith 15" singles on top of each other. (More or less.))