Okay, the factor of the subs facing each other doesn't increase the cancellation factor. In fact, the best locations that were found in the often-cited Harman study on multiple subs were directly opposite each other at the mid-points of the side walls or the front and back walls. An unavoidable partial cancellation effect occurs when two subs(or two of any speakers)play the same sound and the subs/speaker cones are separated by one-half wavelength of that sound. For example, if the speaker drivers are 10' apart, a partial cancellation would occur if they simultaneously played a sound around 56Hz(whose wavelength is about 20', calculated from 1130/56). So it's the distance of separation that's significant, not if the subs are facing each other.
