It isn't about max SPL, but being able to reproduce lower frequencies with at the same output level as the upper ones.
Just because you have the same surface area of cones doesn't mean you have the same displacement. Lower frequencies have longer wavelengths, and in order for a driver to be able to create an equal amount of pressure for a lower frequency it needs to push just as hard as it does for a higher one, but move for a longer period of time. That translates into excursion. Larger drivers have larger maximum excursions.
More drivers, placed to interact with more of room will definitely get smoother response throughout the room, but you need to have the same total displacement to create the same sound pressure levels. (See infinite baffles: a large number of smaller drivers, using the total of their smaller excursion to produce very low frequencies.)
Personally, I've gone from one 12" to dual 8s, to purposefully lose the ability to reproduce infrasonics, because I live in an apartment building and the neighbors were complaining. So I moved the tactile frequencies into a device designed just for that, and let the "subs" handle only the lower, audible bass. (Once I get moved into my own place, I want to be able to pressurize the room 5 times a second to levels of 120 dB.)
