Two Amps One Speaker Issue - Plinius & Rotel

I have two amps a High Current Plinius 8150 (2 Channel) and a Surround Rotel RSX 1065 (5.1 Channel). I have shared two BMW CDM 9NT speakers between them. Both systems are powered through a Monster Power Regulator (HTS 5100 MKII). I can play music using my Plinius Amp without an issue (Monster shows 1.2 Amps output and 120V Input). However when I try to play music using My Rotel, I see the amperage on the Monster going up gradually up to 18 Amps and Voltage drop bellow 110V and the Monster switches power off.

Now the interesting observation is the two amps work like a charm when I connect only one of them to the BMW speakers at a time (Rotel takes only 2.6 Amps).

I see the same Voltage drop even if I bypass the Monster and connect my Rotel Amp to the mains supply.

Connecting and Disconnecting speakers is not something I want to do each time I use one Amp. So do I have a solution to this issue other than buying another set of speakers?

What could be causing this High Amp suction when both Amps are connected to the same speaker? I have got few people to check cable connections and all confirm no issues in connections.