[jphackworth]

This is similar to some of the effects of entanglement in quantum mechanics, where you can't affect the probability of a single event, but you can guarantee the events are not independent.

(spoiler alert)

Once you figure the guesses have to be based on the coin flip you can see, it's pretty straightforward because there just aren't that many possible strategies. Sam always guesses that the coin he can't see is the same as the one he can see, whereas Dirk always guesses they are different. If they are the same, Sam is right and Dirk is wrong. Otherwise, Dirk is right and Sam are wrong. Each is right 50% of the time, they just ensure they are never right at the same time.