Preventing audio feedback in an underwater multiway intercom
Choose two different carrier frequencies, one for the right-left link, and a different one for the left-right link.  So the left-side box transmits on frequency 1 and receives on frequency 2, while the right-side box transmits on frequency 2 and receives on frequency 1.  Choose them so that they're reasonably far apart (think several times the bandwidth of the FM signal), and not harmonically related (i.e., don't make frequency 2 equal to some integer times frequency 1).

If you're lucky, you'll be able to filter out the signal coming from your microphone electronically.  If you're not, then your local speaker will swamp out your local microphone, and you'll need to acoustically isolate the speaker and microphone, with careful case design and with acoustic filters before the microphones -- and you'll need two filter designs, one for each frequency.