Wed Nedarim 73

The Gemara has a Shaila if you can be Maifer the Nedarim of his two wives at thee same time. Or do we say that the Torah says "her Neder" and not her with someone else.

The Gemara brings a proof from a Sotah that you may not perform the ceremony on two Sotos at the same time. Tana Kama says for perhaps, one of them would admit to her sin, but gets more confidence to deny her sin when her friend is also denying. R' Yehuda says: this is not the reason, rather because the Pasuk says to take "her" out, and not "them" out.

The Ran brings those who explain: so we see it's a Machlokes if we make this Drasha, therefore the Halacha is like Tana kama who says that we don't, therefore he can maifer two wives at the same time.

However, the Ramban explains: everyone holds of this Drasha. The Gemara in Sotah explains Tana Kama as R' Shimon that explains the reasons of the Pasuk, why was the Torah care to take only her out. Similarly, we use this Drasha that one can't Shecht two Para Adumos at the same time, since the Torah says you Shecht "her." Also, we use this Drasha to say later, that the man needs to know which person's Neder he's Meifer. (he can't be in doubt if his wife or daughter made it.