They way I've seen it, the manager's manager formally makes the call, but they wouldn't really know the other person involved (they would just work with their s

They way I've seen it, the manager's manager formally makes the call, but they wouldn't really know the other person involved (they would just work with their subordinate managers) nor the details of the particular problem. If they care, then they would get into the details and perhaps fire whichever of the two is actually at fault; but if they don't care much (which I've seen happen, especially in more lower paid/mass market positions), they would rubberstamp whatever HR says - especially since it's not that unlikely that the conflict will escalate into a complaint (no matter if real or not) about discrimination or workplace abuse, and so from the perspective of that manager's manager it's very important that a Proper Process gets followed and (for many jobs, though usually not for IT jobs) not that important to them about who stays and who leaves afterwards, as HR will just get an equivalent replacement.