This could be a debatable point as you posted. Frequency waves generated by both subs would have identical wavelenghts at the same frequencies. By placing 2 subs in the room, the addition and cancellation could be extrapolated to the combined waves. Each sub could either cancel, or reinforce the others frequencies. I would think this would require careful placement to eliminate this.

Perhaps a front set offset from each other, or front and rear offset pair. It would an interesting experiment!