I had the exact same situation happen to me, except it wasn't until over a year later that I put it all together, again without them telling me. I tried to forgive my best friend, and even kept having sex with my girlfriend after breaking up with her, but both of those things ended up being very bad life choices, and wasted time I could have spent building healthy relationships. Neither of the two really had any respect for me or cared much about me, and both tried to lie to me repeatedly until I had unequivocal proof of what had happened. That kind of reaction to the situation should have told me right away I need to cut them out of my life.
  In your case I'd say they both have a moral obligation to tell you for different reasons (hard to compare which one is more or less, especially without knowing much of anything). I would consider it further dishonesty and total lack of respect for you that you haven't heard anything yet.
  I would say that you first ask them about it to see if they would be honest with you, before telling them both that you know. Try to plan it out so they can't talk to each other. You should definitely hear the story from both of them, without them talking in between, so you know if they are really telling the full story. One thing I experienced is that even if they admit to it, they will try to minimize the extent of what happened, and this is easier if they can collaborate without you knowing.
  I think the most important thing in these kind of situations is knowing if this was a spontaneous, one off thing, or if both of them don't really care about your friendship/relationship on a fundamental level, which would be evidenced by the way they handle being confronted about it.