she or he has simply to decide whether opening the door to the captain or FO is reasonable. Independent of what the Pilot Flying is doing is there a sensible reason to exclude the other pilot Y/N?
On what basis does he/she do that? There might well be a sensible reason to exclude the other pilot, but the CC member quite possibly has no idea what the interaction has been between the two pilots in the minutes/hours before one of them left the flight deck..and before it's explained to them the TCAS RA kicks in....( and I know I'm playing Devil's advocate here)
People skills, is supposed to be what CC are trained for isn't it?
Ummm...There are people skills and then there's the skill set needed to spot a crime about to happen.
For the sake of interest seeing as many seem to like the idea of "two up" all times and are happy with one being CC can I ask what are the proposed rules (because there have to be some)..always let any pilot back on to the flight deck? In all circumstances let the captain back in? Only let F/O in if captain agrees? Let any pilot back who gives a cheery wave....etc