Download multiple formats at the same time

[06Games]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.09.20
• I've searched the bugtracker for similar feature requests including closed ones

Description

It would be convenient for streaming to be able to download multiple formats at the same time.
I will take the example of France TV but I think it can be used for other sites:
The download of the live from France TV (e.g. https://www.france.tv/france-2/direct.html) does not work correctly because of this inability. The audio and the video being separated, the audio is downloaded after the video, which, in normal times does not disturb but during a live, it is important to download them at the same time (for obvious reasons)
The download works perfectly if have used the following ffmpeg command:

ffmpeg -y -headers HEADERS -i VIDEO_URL -headers HEADERS -i AUDIO_URL -bsf:a aac_adtstoasc -c:v copy -c:a copy -f mp4 output.mp4

So it would be nice to be able to use this command inside youtube-dl

[06Games]

Hello @dstftw, could you tell me which issue did I duplicate? Because despite my research, I could not find an issue seems