I have a Yamaha RX-V2500 receiver. I have another amp connected to its
Zone 2 output so that I can send audio to another room. But the only
way this works is if I turn the REC OUT/ZONE 2 dial in the front of the
receiver to select a specific source.

I want to leave this dial on SOURCE/REMOTE so that whatever source I'm
currently playing on the main speakers will always be sent to Zone 2,
but when the dial is set to SOURCE/REMOTE, the receiver never sends any
audio to Zone 2.

What am I doing wrong - how do I get the receiver to automatically send
to Zone 2 whatever it's playing on the main speakers?