My girlfriend live-streams games on Twitch. I want to live-loop music (guitar, keyboard, electronic drums) in the background that plays along with her stream.
I want to be able to hear her computer audio (game sounds and stream sounds) in my headphones as well as my music and our voices, all with the lowest possible latency.
Currently she has an audio interface (M-Audio Mobile Pre) with a microphone connected to it, plugged into her computer. My current plan is to buy a second audio interface, into which I will plug all of my instruments and my microphone, and plug all of that into my iPad or my computer. I will then output the audio from my interface into her interface. Then I will use a splitter on her interface's headphones out, and we will both listen to the final audio. I hope that by doing all this with cables, the latency will be low enough.
The only issue here is that we will hear exactly the same thing. Ideally, I would be able to adjust the volume that I hear separately from the volume that she hears or the volume that is output onto her stream. In particular, it would be best if I were able to make the music louder for me than for her, and also if I can hear her voice without hearing my own, and vice versa. The splitter on the headphone out makes all of this impossible.
I have not been able to figure out a better setup which would grant me that flexibility. What would be the best possible setup for this?
Edit: One thing I was thinking was that I could use a line-out from her audio interface that plugs into my audio interface, which would send only her desktop audio and her microphone, and then I could listen using the headphones jack on my audio interface. But since this would be a loop, it presents two problems: (1) How would I make her audio interface output only that audio, and not also the audio that I am inputting into it? (2) How would I listen to that audio on my audio interface without also outputting it?