I'm going to ignore the nonsense posted previously in this thread and try to address the "original post."
Dual subs will provide more consistent frequency response across a larger listening area. A single sub can be EQ'd for a flat frequency response, but that EQ will only apply to a single listening position. That EQ will likely make most other listening positions worse.
Dual subs will add headroom, and that headroom can be used to lower the gain of both subs. This puts both subs into a more linear, lower distortion output range. IOW, they'll SOUND better.
These differences will be realized no matter how large the listening space, (assuming normal sized residential spaces.)
It's not just about "room gain." It's also about the volume of air to be moved. The entire volume of air in the open space needs to be moved in order to "pressurize" the space with sound pressure. A single 12" sub will need to work very hard to pressurize a large space. Adding multiple, larger drivers has the potential to move a lot more air than a single smaller driver.
If one only desires moderate levels, and one only has a single listening position, one can get away with a single sub in many instances. OTOH, if one desires high quality, high SPL bass, with deep extension, in a large space, it generally requires multiple, quality subwoofers.
So say if I have a 1000 cu ft room, and one sub, lets say a SVS PB2000, will give me nice tactile bass. Does that mean that if I move into a 10000 cu ft room, I need 10 PB2000s to achieve the same tactile bass I was getting in the 1000 cu ft room? Would I be able to get away with less subwoofers (say 4) and still have the same amount of tactile bass as in the 1000 cu ft room if the volume is at a sane level?