An option would be if the TV has an analog audio output, which can probably be controlled with the TV's remote. Then you'd split that to both receivers and control the volume with the TV's remote. Inelegant, but that could work.
Maybe. Probably not. That gets deep into the details of the receiver. As I consider it, it probably can't be done with a digital input, since most receivers won't output analog from a digital input, which would be the main way I can think of how you possibly could do it. Even just adding an...
Do you mean you're sending the audio from the TV to the receiver via hdmi ARC? The TV probably has an optical audio output as well, so if the other receiver has an optical input, you can use that for a second audio feed to it.