Steve House is right. Changing the record levels will do nothing useful. Once you boost them back to the correct level, you're right back where you started.

The OP's own solution is the best. That's the way it's handled by the pro postproduction studios. Reducing the unspoken mic is the best solution unless you have access to gates. Even if you have gates, they require patience and careful adjustment to be effective.

Reversing polarity may or may not work depending on the mic placement, frequencies and acoustic environment. I mention acoustic environment because in most outside spaces or large acoustically treated inside spaces, the "tunnel effect" is lessened because of fewer reflections. Think about treating the space. Reduce the reflections. Improve the audio.

Regards,

Ty Ford