The point about two subs "eliminating peaks/nulls" shouldn't be taken too broadly(and maybe was intended in a narrow sense), although that configuration can have benefits in reducing some peaks. When two subs would be located opposite each other either at the middle of the front and back walls or the middle of the side walls they would minimize the primary length mode(side wall position)or width mode(front/back wall position)since those modes are at a minimum in the middle of the wall. While the length mode is at a maximum(front/back)and the width mode is at a maximum(side walls)in those positions, they're of opposite phase at the opposite walls and therefore tend to cancel each other. The net result is that the primary axial modes for both length and width would be minimized, although there are of course other modes that remain.
As to nulls, that's actually a disadvantage of two opposite subs at a certain frequency. Where the drivers are about a half wavelength apart and play the same sound a partial null results. For example, if subs on opposite sidewalls had their drivers about 12' apart, that would be a half wavelength at about 47Hz(560/12)and some cancellation would occur in that area.
