xvfb + chromium + ffmpeg + alsa dummy cards - simultaneous & separate screen+audio recording

I need help in setting up scenario where we have a server (debian preffered) with xvfb + chromium + ffmpeg + alsa + pulseaudio installed. The goal is to record audio and video of provided website(s) urls. Each recording will spawn it's own xvfb and use separate audio channels to grab it's sound (assume we are recording two ore more different youtube videos). Mostly it's done but... HELP NEEDED with sound setup: I have most of that done the help needed is with alsa chromium and ffmpeg setup as I don't know how to tell chromium to output to one of the 10 Loopback cards and then ffmpeg to record audio from this specific card. Each xvfb will occupy it;s own loopback card in order to separate audio streams.