One person giving instructions has been in the laws of the game for many years. Originally only one occupant was allowed to stand at any time, but the professional game pushed to allow two standing, which was then approved by IFAB.
They left the "one giving instructions" requirement in the law at that time.
Competitions can now make their own rules re the use of the technical area, but the law remains as one "giving instructions" at any time.
For those who carry out 4th Official duties, one is more than enough!