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.
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