One or two subs

Unfortunately I have one of those living and dining room combos. 12x26 with 10 foot ceilings.

I think I would go with two subs separated into two different corners or sides so the room if possible.

A 50 Hz sound wave would be a bit less than 23 feet long, assuming the speed of sound is 1125 feet/second at sea level.

A half wave would be 11 to 12 feet long. If your subs are 11 to 12 feet apart, the peak pressure from one sub would reach the other sub as it is making its low pressure point, cancelling its effect at that location, assuming a mono signal.

If you are at some arbitrary listening position in the room, the sound wave cancellation would depend on the frequency and your distance from each sub. For a mono signal, the best positions would be equidistant from the two subs.

Off the center line, there will be frequencies where the two subs will have an additive effect, resulting in a fairly uneven frequency distribution.

Room resonances further complicate the situation.

I frequently read here that 2 subs are better than one. But the part about cancellation in a 2 sub setup makes a lot of sense to me.
I have 2 subs near the corners on one short wall in a rectangular room that is 13.5 x 23 x 7.5

The subs are about 9.5 feet apart. So is Jameswei correct? He states "the output of two subs will likely cancel at certain frequencies unless they are fairly close together."

Whether you have one, two or more subwoofers you will have cancellation/reinforcement from the waves reflecting off of the walls, floor and ceiling. The various nodes of either reinforcement (where the crest or trough of two waives meet) and cancellation (where the crest of one wave meets the trough of the other) are most severe with ONE subwoofer. Having multiple subwoofers creates many more such nodes so that any particular listening spot is subject to many of such interactions so that any one interaction is not as severe in effect. This is the primary reason to use multiple subwoofers--actually creating many more of such cancellations and reinforcements instead of fewer and more severe wave interations.