I suppose you could take the line level output for the sub off your first receiver, split it and connected one leg to your left channel in on your second receiver; then run speaker wire from the left channel speaker jacks to the second sub. Over all I wouldn't recommend it. Its pretty kludgy and unless its a high end receiver it may have issues trying to drive a sub. You would probably be better much off looking for a second-hand mono amp.
Most receivers don't have the power supply to deliver peaks required for the sub. Also, it is usually difficult to find a dual 8 ohm sub which is the impedance that most receivers are most compatible with.