Just a quick explanation...
If you connect two receivers to one set of speakers, you have speaker wire running from receiver A to the speaker, and then wire from the speaker to receiver B.
This connects the outputs of receiver A DIRECTLY to the outputs of receiver B.
This is VERY BAD!!!! It won't make any difference if one is turned off.
I was in the middle of testing several receivers and speaker combinations late one night, got in a hurry, thought I had pulled the connection off one of my receivers, and did this by mistake.
Turned on one of the receivers and blew two 250v 4a fuses in the output circuits in less than a heartbeat. If the circuit hadn't been fused...woulda been ugly.