Concerning
Kiddushin that is not given to the possibility of 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.