One or two subs

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.

James' analysis - tho correct in a vacuum - has it backwards.

Reflections off of the walls of your room will cause destructive cancellation to the long wavelengths of low frequencies - whether you're using a single sub or multiple subs or a full range loudspeaker, for that matter. (You can google 1/4 wave cancellation to calculate the frequency (ies) at which this cancellation occurs.)

So, you always start out with a problem when you have low frequency wavelengths in a confined space. The trick is that - per the calculator I mention above - you can manipulate the details of the response interactions by changing the location of the low frequency driver relative to the walls.

Of course, that's tough with full range speakers. Move the woofer and you move the mid and tweeter. The best position for the woofer is usually not the best position for the other drivers - hence the first advantage of using one or more subwoofers instead of a full range loudspeaker.

The oft-cited benefit of using a second sub is that careful placement of that second sub, relative to the first one, can re-inforce the diminished frequency band created by room interaction with the first sub. That is, place the second sub closer to the wall than the first sub and the bass response trough rises in frequency. Optimal placement of a second sub will almost always result in smoother FR than a single sub, because they can re-inforce the output in each others' null area. A third sub is even better, a fourth better yet, and so on. Take a look at the Audiokinesis web cite for a good explanation.

One complicating factor is the software in the AVR. If you're using room correction software, the AVR will do much of the FR smoothing work for you and the benefits of multiple subs may be reduced. I can't comment on the gear you're using, but my general rule is: without room correction, two subs generally are better than one. With software, generalizations are tough.

Good Luck.

Marty is right that room resonances will be largely determined by the room dimensions, regardless of the number of subs. Careful placement of the sub or subs can help a bit, but the walls will still be the most important factors. (Like, it's very hard to reproduce a sound with a 30 foot wave in a 26 foot room, no matter where the sub is or even if there are multiple subs. On the other hand, the room will tend to emphasize sounds with 26 and 52 foot waves.)

Using multiple subs to try to reduce the impact of room resonances is difficult and not very effective, especially since the output of the subs will be reinforcing or cancelling at different frequencies as a function of how far apart they are and where they are relative to the listening position, as well as dealing with room resonance factors.