Assuming that the main receiver is surround sound you would take the sub out of that receiver, split it into 2 with a Y connector like this: http://www.amazon.com/DB-Link-Shielded-Competition-CLY2...
You can connect this to any input on the 2nd receiver that is line level (any one other than phono).
If the sub receiver is not two channel then set it to stereo (no subs). Connect the subs to the receiver front speaker connections.
You will have two places to adjust sub level (sub out on main receiver, main volume on sub receiver).
If the main receiver is not surround sound you have two options to feed the signal to the sub receiver. First is if it has a preamp output. That would feed the sub receiver directly with an audio cable. If it does not have pre out you will need one of these: http://www.crutchfield.com/p_142LOC80/Scosche-LOC80.htm...
Connect it to the same speaker out that the bookself speakers are connected.
Once you have the speakers balanced you use the volume control on the main receiver.