Yeah, if the amps are balanced (differential) you cannot simply wire + to + and - to -. The REL method is one sub to each amp, using whatever connection method you wish. (I use bananas with spade speaker connections) If balanced, simply leave the negative lead disconnected. If there is hum, ground this lead to a chassis ground. Some amps, such as Pass, provide a ground post for just such a connection. If you still have hum you will most likely have to use the low level inputs from a pre-out or Sub-out on your preamp. IF your amps are not differential amps, your connects are as Miller states.
Subwoofer/monoblock connection
I am relatively new to this and not technically inclined, so please accept the appropriate disclaimers and apology if this question is silly. I have a pair of monoblock amps that I would like to connect to the high level inputs on a pair of subwoofers. How can/should the speaker cable and subwoofer cable both be connected to the single set of binding posts on each amp? Thanks and kind regards.