[youtube] -f to merge video with two audio tracks

[Gorrrg]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.03.24
• I've searched the bugtracker for similar feature requests including closed ones

Description

There are surround sound audio tracks popping up on YouTube now, and I'd like to mux both a stereo and a multi-channel track into the file using something like --format 313+258+251, so that the resulting file can be played on both stereo and surround setups without sounding bad.

But that gives the error:

[youtube] fbyIYXEu-nQ: Downloading webpage
ERROR: The first format must contain the video, try using "-f 251+258"
ERROR: 'NoneType' object is not subscriptable

Example video is the Vsauce Mindfield series: https://www.youtube.com/watch?v=fbyIYXEu-nQ

The example formats would be:

251          webm       audio only tiny  147k , opus @160k (48000Hz), 21.18MiB
258          m4a        audio only tiny  388k , m4a_dash container, mp4a.40.2 (48000Hz), 63.82MiB
313          webm       3840x2160  2160p 18419k , vp9, 24fps, video only, 2.39GiB

And I expect ffmpeg to mux them into a Matroska file. I know I could download the individual tracks and then mux them together manually, but I like how youtube-dl is able to embed thumbnails, metadata, subtitles and descriptions in one go.