Realistically, what are the chances that you are going to have a technical foul on the assistant before the head coach shows up? Not much, I don't think...
And if you do, fine, he gets a direct, and when the head coach gets there just let him know he already has an indirect!
I understand that "technically" there can only be one coach, but I think that the spirit and intent of the rule speaks more to not having two coaches at the same time -- not the head coach being late for a game.
If I officiated all my games "technically" I would bet that over half of them would start with administrative technicals for the book not being ready. Personally, I just don't like to start games off on a bad foot!