OK I faffed around a bit more and have figured out a way to achieve what you want (I think). Firstly, when creating your multicam sequence you must use a preset with multichannel (rather than stereo) audio. Otherwise your multicam sequence can only have 2 audio outputs. Group your cameras & sep sound (I had 'all audio' selected). Open the resultant multicam timeline as a timeline and rearrange audio so that the correct audios are on the correct tracks (for my test I had an 8 channel wav file on first 8 tracks then 4 stereo pairs from the cameras on the next 4 tracks) - and delete empty audio tracks. Open audio track mixer and route the audio correctly (e.g. in my example A1 should be panned left and routed to outputs 1&2, A9 was a stereo pair routed to outputs 9&10) Modify audio for the multicam clip so that it reflects the correct number of audio clips (16 mono in my case) Edit that mulitcam clip into a timeline with correct number of audio tracks - I now had 16 (8 x mono from the dual sound and 4 x stereos that appear as monos) tracks on the timeline, with 16 different audio tracks, all showing a sync relationship with the multicam clips, that I could then edit. Flattening *appeared* to produce the correct results (with the caveat that more than one audio clip in a multicam source timeline track does lose clips when flattening). Yet to use this in anger though....
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