It's only my opinion, but it just seems clear to me that there's only one head coach. Just look in the game program. If the head coach is not present, that doesn't make the assistant a head coach. He's just filling in until the head coach arrives. Likewise, if the head coach is ejected, the assistant who takes over does not become the new head coach, with coaching box priveleges. It seems to me that a team's head coach is not just whoever happens to be calling plays at the moment. It's the team's
head coach. And there's only one of those.
The coaching box may only be used by the head coach (FED 1-13-2 NOTE; NCAA 10-11-1). And there's only one of those per game.
The only other rule citation I can offer is from the NCAA rulebook. It's AR 15 on page 134 of this year's book. It says basically that if a team has co-coaches, only one of them is allowed to stand. The intent is pretty clearly that only one person (per team) per game gets coaching box privileges.
(The real beauty of this AR is that even tho only one co-coach is allowed to use the coaching box, they both get charged with any indirect T's!!
)