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.
I am sure airlines and countries / regions vary.
The systems fitted to our aircraft cover your requirements I have underlined. If these systems are mot available, then we use a second person as you suggest.
If we have these systems
and require a second person, then the second person can only be there to "baby" the remaining pilot. Or for political /
PR reasons
And this adds another dimension to the issue:
A JetBlue flight bound for Las Vegas had to make an emergency landing in Texas in March 2012. The co-pilot locked the captain out of the cockpit as the latter was behaving erratically. The captain then had to be wrestled to the ground by passengers.