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

Duke is certainly more qualified than I, but let me add that unlike the dedicated sub channel in home theatre sound, summing stereo bass into one channel can actually subtract some bass signals.

More is better unless you can't afford them or don't have room for them.

2 is better than one, especially if you can locate each in the optimal locations, which is usually not symmetrical.