Hmm - still not sure if I would T up the coach or not in this situation. Here are my thoughts, though.
1. How far out on the court is the coach? A step or two over the sideline when I'm also running up-court - I'll pass. Or was he at the lane line extended? Did any of the opposing players have to change their route to avoid him? Did they even have to think about changing their route?
2. Is this going to make the game better? The coach is excited - if I tell him to stay in the box, is he going to realize he was out, or is he going to get ticked at me? Has the opposing coach been out of the box and, if so, what warnings/penalties have I imposed on him?
3. If you choose to T the coach, it needs to be immediately. To wait for A to make the layup and then give B two shots penalizes team B, which should have the potential to make a 2 point shift in the game.
4. As to who should call this - obviously this would be best to have a call from your partner, and it's good that you held off on the whistle because he had a no-call. However, I think that you giving a T would be completely appropriate in this case - it's protecting your partner's a**. I think this would be a great pre-game situation to discuss, especially this aspect - far too often one official has his back turned, a partner sees something, but calls nothing (not that that's happened to me or anything
).