You could simply mux the audio track of that 2nd file into your movie – this way users can already pick from the available tracks.
There’s already an existing feature suggestion asking to support “external audio tracks” (same as external audio tracks). From what I understand their target is to use this when storing their media on cloud storage in order to avoid syncing the full file back and forth. Even if you’re coming from a different use case, this will also work for what you’re asking for here.
I suggest you comment/vote in that thread in order to avoid distracting/dispersing votes.
Therefore I’ll close this suggestion as a duplicate.
