Sort of like a priority vox switcher. I started drawing up a switch box that could do something like what you describe last year. My wife and I were getting tired of
1) Stepping on each other when we both tried to key our FRS units.
2) tired of listening the to cacophony of the CB and FRS and scanner units.
So I drew up a switch box that would route the two FRS mics to one frs unit and interlock them so that once one of us keyed the other couldn't, with an option for "Pilot over Co-pilot" priority.
And the monitor took the CB and FRS and Scanner outputs and gave priority in that order.
I prototyped one part of the circuit using some optical relays and stole the priority circuits off the web (just basic vox circuits.). You are more than welcome to them if you think they might be of any use to you. Since I never built the complete thing I have no idea what the end product would be like.