The Mishna on our daf presents a case that appears to be most unusual. What happens if a man marries one of two sisters, but does not know which one he married? The Mishna teaches that in such a case the man must divorce each of them, since he cannot marry two sisters. Even after he divorces one he cannot marry the other, since she may be the sister of his divorced wife, which is also forbidden by the Torah.
Two possibilities are presented by the Gemara as to how such confusion may have come about.
From ga’onic literature it appears that a case was once presented to them where a marriage took place and afterwards there was a dispute about what happened, to the extent that no one was sure who was truly married. In this case, it was clear that a marriage took place between two people; we just cannot determine which two people they were.
The other possibility raised by the Gemara suggests that the confusion in our story stemmed from the fact that the marriage somehow was done without clarity from the very beginning – perhaps a situation where both sisters appointed a single individual to act as their agent to accept marriage proposals on their behalf. Someone approached the agent and offered him kesef kiddushin to effect the marriage – and said “with this money I am marrying one of the sisters” without clarifying which one he intended.
The Ritva suggests that if we assume yesh bereira – that we can ascertain someone’s intent retroactively, once they make a decision later on – then such a marriage may work. Still, he argues, the case might be where a person leaves the decision to someone else (e.g. “I will marry whichever woman my father decides”) and then that person disappears and cannot make the decision.
In the end, this possibility is rejected because that would be a case of kiddushim she-lo nimseru le-biah – marriage that could never be consummated – since the marriage itself created a situation that does not allow for a marital relationship to take effect.