"RST Engineering" wrote

Y adapters for the headphones are easy. Y adapters for the microphones
are
well nigh impossible.

How about a true Rube Goldberg solution?

Use the Y for the headsets, and make up two boxes (and wires to go from the
front to the back) that has a cord to plug into the intercom and a jack
receptacle and a momentary switch(with a normally open and normally closed)
in each one. Press the button, and you get to talk, and cut out the
possibility of the other mic being hot at the same instant. Make two; one
for each side.

I know, Jim; try not to cringe! (I can see you doing it, right now!) g
--
Jim in NC