If the subs are placed within 1/4λ of the mains at the crossover point, then they will sum very well. They'll effectively be the same acoustic source. If not, then there will be some lobes that form in the room.

The thing is, wall reflections act as sound sources too. Each subwoofer in the room and each (wall, ceiling or floor) reflector acts as a sound source and they aren't going to be within 1/4λ of each other through the subwoofer band (or any other band, for that matter). Interaction between acoustically distant sound sources is nonminimum phase.

The only indoor solution that presents itself is dense interference. Use multiple point sources to average the sound field. CARA is room modeling software that can help you find what positions are best to place your subs.