Is it better to have two matched subs or one

In general, the in-room bass smoothness improves as the number of intelligently-distributed bass sources increases.  So yes, two subs should be a worthwhile improvement over one, assuming you place them fairly far apart and at different distances from nearby room boundaries.  

Duke

@dbphd asked: "unsound/duke, how does [summing stereo bass] subtract some bass signals?"

In theory to the extent that the left and right channel bass signals are out of phase with one another, the resulting net electrical signal would be less than 100% summation. And in theory if the left and right channel bass signals are 180 degrees out-of-phase with one another, their electrical sum would be complete cancellation.

In practice, I don’t think this is a significant issue. I’ve run multiple subs in stereo and in summed mono and in dual mono and have yet to notice a difference which could be attributed to some bass signals being subtracted.

It is my understanding that true stereo south of 80 Hz or so is rare at best. 

Real (or synthetic) stereo bass can offer an enhanced sense of immersion in the acoustic space of the recording. 

Duke