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.
Let me get this straight, you have both amps connected to the same speakers at the same time? A major no-no in my experience. Use some type of external switching box to select EITHER amp; I am surprised you haven't blown at least one up. I am not sure what you are doing but the driving amp will see the amp that is off [ it IS off isn't it?] as part of the load it wants to drive. This is not good! Either get two more speakers, a switch box or change the connections each time.
x2 - you're essentially running power the opposite direction it was intended through your amplifiers.
x3. Never connect two amplifier outputs together.

-- Al
NIles makes a signal sensing box that will automatically switch them for you.
Thank you for your responses.
Solved the issue by feeding the front preAmp output (2 Channels) from the Rotel Amp to Plinius Amp and disconnecting Front speakers from Rotel Amp output. Now Only the Plinius Amp is feeding the Front 2 Speakers. Rotel continues to feed the remaining 3.1 Channels directly to its speakers.

Everything is now stable now and performing as expected.

BTW. Switching an amp off is not an option since Plinius has a 48 hour warm up time!
Glad to hear things worked out for your Weera.

I had inadvertantly connected 2 amps to one set of speakers and it was a DISASTER. I destroyed one amp, the bass drivers in each of the speakers and blew the fuses on the 2nd amp.

Wow Plinius amps have a 48 hour warm up time. Holy Crap! Do you just leave it on 24/7?
Switching an amp off is not an option since Plinius has a 48 hour warm up time!

Sorry,I don't understand this statement.Would you mind explaining why your amp takes 48 hours to "warm up".Just curious.