Concerning Kiddushin that does not allow legal cohabitation, Abaye said: It is a valid Kiddushin; Rava said: It is not a valid Kiddushin.
Rava: “When a man shall take a wife and cohabit with her...” teaches my rule.
Abaye: If a man betroths one of two sisters, and does not know which, she is betrothed but due to not knowing which one is his wife, he needs to give a Get to both. According to Rava, Kiddushin should not even take effect!
Rava answers: The man knew whom he betrothed, thus the Kiddushin took effect, and then they intermingled.
This is the "K" in the six cases abbreviated Y A L K G M where the law follows Abaye against Rava.