Who has the other talkback mic?
Do the talk back mics have a mute switch?
If there dynamic mics a Y cable would work…..depending on the how the
mute switches (if any) are wired on the mics.
If you have channels and mixes to spare use two channels for talk back, send those to a mix and assign the mix to an ME channel.
Use a cheap small table top mixer to submix the talk back mics.
ART makes a passive transformer iso mic combiner.