Using Two Subwoofers with Two 2-Channel Amplifiers.

+1 wolf_garcia regarding a monaural or summed below an approximate crossover point depending on the speakers and the room.

With the exception of subwoofers by REL manufactures include the convenience high level connectivity for those who lack source RCA outputs, experimentation should be seriously considered.

Review the subwoofers manufacture instructions when using high/amplifier level connections. Connecting both amplifiers Left channel outputs to one subwoofer seems unusually redundant.

With an honest respect to zipost's experiences. My only sub installation with very small mains that began their in room measured falling off at 97Hz stereo high level connectivity did sound subjectively better to the owner.

With main speakers falling off below 50Hz monaural has always produced a more desirable effect in my experience. It also liberates subwoofers for the always unique crawl test sub locating and an overall improved performance in speed and room node control.   

Sorry, but just to be clear, when to say "Mono" or "Monaural" you are referring to connecting Left & Right channels to each Sub correct? My initial thought was that this might confuse the Sub, but it sounds like you guys are saying that this is not necessarily the case (".. canceling standing waves..")

Depends on the connectivity options your subwoofers offer. My only high/amplifier level experience was with a REL Studio III using its proprietary cable. Maybe someone else here can answer your question.

By "standing waves" if you're referring those unusually bass heavy locations in your room they can be addressed by experimenting with subwoofer locations and feeding the subs a summed, mono, or monaural signal.

Eventually your setup will be as unique as your room and the system being used.