Is it better to have two matched subs or one

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

@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?

I've read that additional subs help with the bass "in the room" but don't most people sit in one spot? And if that spot has sufficient bass from one sub, what do additional subs add? 

Regarding "stereo" bass, what MC says about the signal below 80hz makes sense, but what if the recording mix has different amounts of signal in L & R? Then, having just one might not be getting what's supposed to be coming through the other, right? Or maybe that's just a red herring, and songs are never mixed so asymmetrically. 

@hshfi Thanks. That makes sense. I have a second sub with my HT system of a different make that I can try. It's a Klipsch while my main sub is a REL. Different calibers. I will try two subs with those and see what happens.