DON'T DO IT!
I've done this. A long time ago. Twice.
With two receivers. First, by accident. Second, purposely.
There are conditions that may allow you to do so with no harm. As long as both are NOT ON at the same time. But without diving into the circuitry with a schematic, one will not know.
The details are clouded but here's the best I can remember.
The first thing I notice is the system will not play as loud. One usually knows how loud their system plays at say the 9:00 position. (btw this was enough to kill the other amplifier) But going on. So, you turn up the volume to 1-2:00 position to achieve the correct volume. Why? I figured that I was feeding power to my speakers as well as backwards into the other connected amplifier. Which it's output devices did not like.
The first time noticing something was not right (the 9:00 position), I just wiped out the 4 output transistors of the Connected Receiver.
The second time I went for the 1-2:00 push. And then turned the Connected receiver ON. OH BOY. A loud burp from the speakers. Flashes. Smoke. It blew just about all the fuses and the outputs transistors of both receivers. Plus the driver circuit board to the output transistors of the Connected receiver. Quite exciting. And destructive.
DON"T DO IT.
