One or two subs

For your application, the single sub might be better.

Since much of the low frequency signal is essentially mono in nature, the output of two subs will likely cancel at certain frequencies unless they are fairly close together.

A friend of mine does audio and home theater installations and used Sonus Faber and REL equipment for such installations. If budget permits, he always went with multiple subwoofers. This is done not to preserve the stereo/multichannel effects, but because it is MUCH harder to get even bass response in a room with just one subwoofer. Unless the room is really big and you like to really crank it up, two smaller subwoofers would be preferable to one really big subwoofer.

Two, for best affect.

I think an important piece of information that you omitted is the size of your room? If it's a large room I'd go with two subs along the line that Larryi mentioned above. So what is the size of your room?

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