[Rangi42]

I made a table of all four possibilities:

    A_coin  B_coin  A_guess  B_guess
    H       H       ?        ?
    H       T       ?        ?
    T       H       ?        ?
    T       T       ?        ?

Then I tried to fill in the guesses, given that A_guess has to be based on B_coin and vice-versa. After making Alice guess the same as her coin flip, since that's a simple thing to try first, it was clear that Bob had to guess the opposite of his coin flip for each row to have exactly one correct match.