פירוש על קידושין 102:16

Each husband cannot have relations with either woman because she might be the sister of the one he betrothed. So if kiddushin that cannot be followed by betrothal is not kiddushin, then why is either woman betrothed? This is a difficulty against Rava.

Again, the Talmud resolves the difficulty by saying that at the point of kiddushin each man and each woman knew who was betrothing whom. The kiddushin were valid at that point. Then they were mixed up.