Yeah it still took a minute. The way I realised it was Bob is covering them for the same flips, so if he flips H, he covers if Alice flips a H too. But if she flips a T, Bob is no longer covering her, but she knows when Bob isn't covering her, if she guesses for the opposite, she'll cover them. 100%.

The next level is: Eve joins the game, and the coin gets another side.