this guide exists because i'm too lazy to walk anyone through this, so i'll be trying to explain what i'm doing and why, so if you hit an issue, you will at least have some idea of what you can google to try and fix it.
but, after a lot of youtube videos and forum lurking, i think i have a setup that works pretty well. So i knew OBS was pretty nifty already and probably had the capability to fix this problem, but guides in other places felt either semi-incomplete or unnecessarily complicated.
editing said footage can be tricky, though, when you just want to hear the game, but your friends were talking a lot, or you were blasting nightcore, or you were watching some stream on the other monitor, leaving your recording semi-ruined. I use OBS to record epic gameplay with me and my pals.