My understanding, co-location creates a synergistic coupling of sound waves. The benefit being, a 6dB increase in SPL.
To verify this, one can take measurements on two of their subs, one sub turned on at a time. In the simple, turn one sub on, take a measurement, turn the first one off and the second one on and take a second measurement and then turn them both on and take a third measurement. This is an easily verified, naturally occurring phenomenon with each room's acoustics lending it's own damping factor to the above.
My understanding, if each of the subs are within one wave length of each other, this co-location will take place. This means the lower bass or longer wave lengths will take benefit of this natural phenomenon. The longer ones outside the co-location placement distances, won't.
(well that sucks)
What that means, if your subs are ten feet apart, all bets start to shred a bit as a 120Hz sound wave is about 9.4' in length. In our case, we have three subs on one wall, the distances between subs is asymmetrical so depending on the distance each sub is from the other, the co-location boost varies. Which technically makes for personal room related problems and a 3dB drop can cause a measurement graph some consternation.