It would be helpful if you could select a different audio channel for the streaming settings for each of the 3 streams.

Stream 1: Channel A
Stream 2: Channel B
Stream 3: Master

So the same program could be set in different languages. In sports, e.g. With comments in different languages.

You can perhaps try using the advanced setting where you enable multiple instances of vMix to run at the same time.

Yes, if your hardware can handle it. You can run 2 instances of vMix, then you could stream to 6 different streams in theory... Never tried it myself on the same PC. Maybe Martin or Tim from vMix could demonstrate this.

With 2 computers with NDI each running vMix. Feed 1st Audio into main computer and send video from main computer to second computer via NDI and feed 2nd audio feed into second computer, then stream from both computers.

+1

For now, you can install standalone fmle from the adobe website, and use a 4k vmix version, where you connect ext.2 to fmle, which you run in the background, while also streaming from vmix via ffmpeg. This way you can set another audio configuration on the external 2. Beware though: fmle uses a lot of cpu.

what i did before is this

stream left right channel with different audio

then

a. let the user decide left or right or
b stream two instances from the master and select left or right on the second and third vmix machine.

a is more simple.

I always see a stream as second and the recording as first. so i produce for ondemand. having a left right audio channel is easy for post so i can produce left and right output very quick and easy.

if your stream is the main: then don t try to put it all on one machine. a simple basic hd of vmix will do and a simple laptop will be ok for streaming...

keep it simple
stefan