Question / Help Can't get microphone+desktop audio working

[Fabian]

On Windows, I had these settings: Desktop audio channels 1 and 2, microphone channels 1 and 3, stream channel 1, record channels 2 and 3.
But on Linux, no matter what I do, I can't get both microphone and desktop audio to record. Apparently I was able to stream it together, because people responded to me, but I can't record it. Sometimes the desktop audio device isn't even found, sometimes it's completely silent, sometimes I hear nothing from the microphone (like the entirety of my 27 hour stream recording), etc.
How do I set OBS to record microphone and desktop audio to separate channels and stream both?

 

[Narcogen]

I doubt this has much to do with any setting in OBS and a lot to do with whatever distro you're using and what audio system you have installed.

 

[Fabian]

Debian 9.5
I don't really know how I check that audio system thing, but I think it's "Alsa".
But why would that mean that I can't record microphone and desktop audio at the same time? Especially since OBS apparently did record both, since in my local file I have desktop audio and in the stream I had microphone audio (probably also desktop).