I need to mux two separate fmp4s, one with just video and one with just audio. I expected to just need to generate a new init segment combining both tracks info from the separate files (i.e. initSegment([...probe.tracks(video), ...probe.tracks(audio)])) and then interleave the moof/mdat boxes from the two files unchanged, but the resulting file didn't work. Can you give me some ideas what's actually needed for this task?

As a side note, the audio codec is mp4a.40.2 according to both mpc-hc and the js call to addSourceBuffer that loads it but when i call probe on it the result says mp4a.40.5, could that be an issue?