This is not quite your situation, but it may contain some pertinent advice: https://forum.audiogon.com/discussions/how-to-connect-monoblocks-to-a-single-rel-sub-with-one-rca-input
Help Connecting Dual Subs To Monoblocks
I have Pair of Audio Mirror Reflection monoblocks. I’d like to connect a pair of Rel subwoofers. Hoping someone hear know if this can be done safely with these amps. I’ve asked Audio Mirror and haven’t got a clear answer as jpg yet. I only gave one set of pre outs on my preamp so I need to use the high lever connections. I’ve researched some and I’m left uncertain if it would work or possibly damage the amps or the subwoofers. Thanks in advance for any help.