Some years back, at Ford, we measured the LF signals for about 100 CD's. I believe that every one of them, or at least virtually every one, had mono LF signals. There is simply no way to distinguish between a mono LF signal and a stereo one so why not have multiple speakers share the LF load. Now if the audio producers figured out the advantage of a decorrelated stero LF signal things might be different, but then when summed to mono, the signal would be degraded.
No, the right thing to do is to send a mono LF signal, and then decorrelate this signal between several sources on playback. That way there is no degradation in the case of a single sub.