[Freddy]

Don't have books here at work, but look under "Head Coach's Rule", which has prompted me, when R, to ask, if two or more coaches are there at the initial greeting, "Which of you is the head coach for purposes of using the coach's box?" That seems also to have served as a tactful suggestion that the other's aren't, by rule.
I had two guys who were twins who were dressed very much alike who responded, "We share the duties; I take the first and third quarters, my brother takes the second and fourth." My partners and I kinda stood there, dumbfounded, before finally declaring that one of them had to be appointed the head coach for the game.
We have a head coach for an area girls' varsity team who prefers to sit down the whole game and have her assistant use the box. A technicality, perhaps? We simply declare the assistant the head coach for the game and all goes right with the world.
In districts this past year, we nipped the assistant-coach-standing-in-the-box in the proverbial bud by saying very early in the contest, "Coach, your assistant cannot be standing--only you have the coach's box." To which he, surprisingly, given his status as a veteran and long-time respected coach, asked, "You mean for the whole game?" I just gave him the blond deer in the headlights look and his assistant complied the remainder of the game.