They are not redundant and fail safe.
One sits at the front & controls the driving.
The other sits in the middle of train, 5 cars back & controls doors.
Nothing about having the two of them creates redundancy.
The train is not moving or being driven when the doors need to open and close. Why can't one person do it? I am sure there is an interlock, but this makes the tasks sequential, so the doors must be closed before taking off.
Yes, that's my point. One person very much can do it. There are train lines that have their signals & stations setup for this, and the trains prepared for this mode of operations. The MTA keeps caving to the unions so that the trains remain staffed with 2.
What's more although the train operator (driver) can open and close doors, the conductor (doorman) is not certified to operate trains and thus provides no emergency redundancy except perhaps in terms of crowd control.