The Binding of Isaac Multiplayer is available in Rebirth and Repentance, but they are wholly different things. In the first case, you can cooperate with a friend who is sitting near you. It makes the play easier and much longer: when one baby dies, the other continues pushing forward.
In the second case, friends can play remotely. It is so useful for the cruel world of Repentance because a person alone is almost doomed to failure there. Plus, together, you take bad luck with a portion of optimism!