Everyone is missing the point of having two in the cockpit. In the US it is required to visually confirm who is entering the cockpit and control the door at all times. The second person is there to visually check the person entering and immediately close the door if open and someone moves near the door. The lessons of 911 never really hit home in the rest if the world. It's a smart policy.
It has absolutely nothing to do with having someone up there to watch the remaining pilot.