Two amps into one pair of speakers

A Disaster waiting to happen and I doubt you will have to wait very long.

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.

I think you meant to say stepping the signal voltage up. Not down.

I also would think the secondary winding of the output transformers, (of the amp that is turned off), would add a parallel impedance load to the output transformers of the amp being used to drive the speakers. (Speaker coils + secondary windings of output transformers of the off amp.)

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Either, or.

No need to build custom, use  a Solupeak P2.

There is no sonic benefit to running 2 amps at once and dangers galore.

Neither amp is designed for Armageddon.

Unless one of the amps is rated for above 100W @ 8 Ohms (not unlikely...McIntosh often are), the device suggested by fuzztone is perfect to resolve this dangerous situation.  But note, the PL should be powered off when not in use as the switch presents an open circuit when deselected, and tubes don't like that.  So this is not a good on-the-fly A/B comparator, just a disaster avoidance method.  If the Mac is a very manly model 200W, 300W, 450W...even 600W, then the "friend" should just get banana plugs for both cables and NEVER leave them both connected.

"Accidentally having them both on at the same time is a great way to create memories."

😄😁😅