Is it better to have two matched subs or one

They don’t have to match, but it’s better if they do. Any difference is more likely to make localization of the subwoofers more apparent.

Contrary to popular belief bass can be stereo. If the bass driver from one channel is moving in the opposite direction of the other channel, when summed they can cancel out the signal between them.

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.