I am becoming despairing of the myopia in this thread.
The unavoidable and necessary truth that exists at the heart of the vast majority of commercial aviation transport is that responsibility for the safe conduct of the flight rests on the shoulders of the captain and FO. The balance between those two is what it is, and not the subject of this post.
The simple and worthwhile objective of the "two person on FD at all times" rule is that it materially reduces the ability of either to usurp the responsibility and authority of the other, so helps to ensure that the conversation between the two can continue on whatever basis applied before one or the other left the cockpit.
That's it. Period. It doesn't alter (give or take fractions) any risk that doesn't already exist.