He should get a switch. The output of amp 1 is present on the secondary side of the output transformers at amp 2. Depending on the configuration, it could be stepping the signal down and getting into the circuitry of amp 2. There is probably a polarized capacitor in there that’s working pretty hard. Even if he can’t get a mapletree switch as another user recommended, a simple switch can be had cheap enough. There’s still a risk with a simple switch - tube amps need to see a load or in some cases a dead short to keep the smoke down.
It also may be useful to measure the signal with and without amp 2 connected. Depending on the topology it could be affecting the load amp 1 is seeing.
Two amps into one pair of speakers
This is a newb question, but my friend has two integrated amps hooked up to his speakers, one McIntosh and a Prima Luna. One is connected with banana plugs and the other with spades. He said this will cause no problems as long as they are not both sending signal at the same time. Is this true. It just seems a little strange to me.
TaterMike