Question / Help Separate audio streams when recording with custom output

I'm trying to do some high quality recordings while I'm streaming. However in the resulting file, I want to have two audio tracks, one for game, one for commentary. That said, I've configured OBS to the mix the mic and game audio into one track, the mic in another, and game in another.

Now when I go to Settings->output->recording and change 'Type' to "Custom Output (FFmpeg)" the check boxes for which audio track to include in the recording change to radio buttons. This indicates that I can only record one audio track at time.

Is there something that I missed in the conf? Did I do something wrong?
 
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