Hello Ed and welcome to the Cow Audio Forum.
As Ryan suggests, an expander. I’d try a gate first, because that’s what I’m used to.
Set a gate on each track that only lets that person’s voice get though when they’re speaking.
Sometimes it works very well. Sometimes you get someone with a REALLY BIG VOICE that opens the gates on the other mics and you have to Do as Bruce Watson suggests and fix it in post.
You might get better results if you use headset mics because they are closer to the mouth of the person speaking, as such setting the gate to keep loudmouths out is easier. Not fool proof, but easier.
Some of the better Automixers by Shure and Audio Technica have direct outputs and the main output does the automixing per the above. You could record the iso tracks and the mixed track on different tracks and then go back and touch up where needed.
Regards,
Ty Ford
Cow Audio Forum Leader
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