Two amps into one pair of speakers

More power = better sound, right? (some people believe that)

I don’t understand why he’s doing this. Pick an amp and use only that one. They’re integrated amps so use the multiple inputs on one.

I also understand that bi amping a pair of speakers can be done with four amps, two per speaker, if the amps are balanced in some way with an umbilical chord. Honestly, I have no idea how this is done but I remember an old time audio dealer explaining this to me. If it were me, I would get a pair of jumper cables and pick an amplifier.

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. 

Accidentally having them both on at the same time is a great way to create memories. You can build a switch pretty cheaply, but need to use a different approach for tube versus solid state amps. Be sure the switch is rated for the current at or above the fuses in your amps  

audio-room.net/connecting-2-amplifiers

This switch can do 2 amps to one speaker or two speakers to one amp