Is it better to have two matched subs or one

Matched means very little with subs. Room placement is more important to low frequency distribution.

I was misinformed by REL who said their high level subs should be identical. 

BASS IS/CAN BE STEREO

bass fundamentals, and their overtones give location, imaging of jazz or rock bass player is easy and enjoyable to perceive when done right.

My speakers have 15" woofers. Think of them as a stereo pair of front firing subs. Excellent imaging of bass players, depending on the engineering of course.

Or, use a matched stereo pair of self powered subs, forward facing, adjacent to the mains to preserve imaging.

Mains: avoid ports or passive radiators, especially side/rear/bottom. Preserve direct radiation, tweeter directed to seated ear height, stereo triangle.

Toed in normally for single listener, toed in more for 2 listeners to achieve a wider center and hear both channels. Aim left speaker toward right chair, aim right speaker toward left chair. Each person is closer to one side, but gets more direct radiation from other side, works!

Preserve perfect positioning: Just mark the inside front corner of your mains, maintain that when changing toe-in amount.

Self powered woofer because if connected to sub, back to amp, it removes the amp’s need to produce low bass, AND the mains also do not try to produce low bass. Allows less powerful amp for mids and highs, and that makes trying tube amps easier.

Adjust crossover, volume, everything more easily doing it’s job.

@elliot Thanks for that helpful post. One additional element of my setup with REL is that these are high-level inputs. Possible difference there, re: Left/Right?

Subs don't need to be identical.  I have two Rythmik subs and two SVS subs.  I have them set up in stereo, but I don't think it really matters.  They're all over my room... in front of me, beside me, behind me, facing into the wall, away from me...  I have relatively small monitor speakers - the KEF Reference 1.  I have no trouble identifying where the bass player is standing on most recordings because they provide all the cues I need and build a solid soundstage with great imaging.  Listening to the alternate version of Fleetwood Mac's Rumours I could picture where John McVie was standing.  Now when I play something like Deadmau5 or Bassnectar, the bass is everywhere.