Note by @fastfinge

@andrew @jscholes @klittle667 So looking into this a bit more, it looks like ffmpeg can just do everything you want itself with amix and amerge. ffmpeg.org/ffmpeg-filters.html#amerge

So use ckucoo to intercept Channel's call to commskip so you'll know when the recording is done, and run ffmpeg to modify the output recording to have audio from both audio streams mixed together. github.com/Channels-DVR-Goodies/cuckoo

Sadly I can't actually do this myself as I'm not in the UK, so I couldn't test it. But once someone does, the process could be easily documented.