I'm unfamiliar with CS:GO, but in L4D2 (with the help of an
extension to enable sourceTV) you'd just use M1 / M2 for toggling between player POVs, and SPACE for switching view modes (first person, third person, free-view). Just recording one demo captures everyone's POV. If that doesn't work for you maybe there's a cvar for recording everyone's POV and not just one person's.