(Mishnah): If a man was Mekadesh one of two sisters and does not know which, he gives a Get to this one and a Get to this one;

If he died leaving one brother, he does Chalitzah to both. If he left two brothers, one does Chalitzah and the other Yibum. If both did Yibum, we do not take [their wives] from them.

(Gemara): [Our Mishnah said that] if a man was Mekadesh one of two sisters and does not know which, he divorces both. If he died leaving one brother, he does Chalitzah to both.

Question: Everywhere, you say that there is no Chalitzah after Chalitzah [for two women who fell from one man], and here you say so?!

Answer: Here [there is no Chalitzah after Chalitzah] regarding Vadai (we know that both fell). Here it is a Safek (which was his wife. Therefore, both do Chalitzah due to Safek.)

(Mishnah): If he left two brothers, one does Chalitzah and the other Yibum.

Question: Everywhere, you say that when we tell him to do Yibum, we do not tell [even another brother] to do Chalitzah, and here you say so?!

Answer: Here [when we tell him to do Yibum, we do not say to do Chalitzah] regarding Vadai. Here it is a Safek.

(Mishnah): If both did Yibum, we do not take [their wives] from them.

(Mishnah): If Reuven and David were Mekadesh Rachel and Leah (sisters) and each is unsure whom he was Mekadesh, each divorces both;

If each died leaving one brother, each brother does Chalitzah to both.

If Reuven left one brother and David left two, Reuven's brother does Chalitzah to both. One of David's brothers does Chalitzah and one does Yibum. If both did Yibum, we do not take [their wives] from them.

(Gemara - Beraisa): Yisraelim and Kohanim are the same for this. (TOSFOS HA'RID (14a) - even though each sister is a Safek Chalutzah, we permit a Kohen to do Yibum. Since Reuven's brother did Chalitzah to both, it is clear that there was a Safek. People will not come to permit a Vadai Chalutzah to a Kohen.)

Limitation: This is only when [Reuven's brother] did Chalitzah, and afterwards one of David's brothers does Yibum. To do Yibum first is forbidden, lest his brother die, and it turns out that he had Bi'ah with the sister of his Yevamah. (If his brother does not die, he will do Chalitzah, and it turns out retroactively that her sister was never Zekukah to the one who did Yibum.)

Inference (Rav Chisda): One who has Bi'ah with the sister of a Yevamah Zekukah to him, he did not disqualify her from Kehunah.

Note: The Yerushalmi (Reish Perek 3) holds that Zikah is mid'Oraisa, and the Torah forbids Achos Zekukaso (OHR SOMAYACH Kuntres Zikah 30, HA'GAON RAV C. KANIEVSKY, SHLITA). Perhaps such a Bi'ah disqualifies from Kehunah. We learn a Kal v'Chomer from a Chalal, whose Bi'ah is permitted, and he disqualifies from Kehunah (Kidushin 78a)! If so, even b'Di'eved a Kohen could not keep her, for this is a Safek Isur Torah. Therefore, Rav Chisda inferred that she is Kesherah to Kehunah. (Indeed, if she were disqualified mid'Rabanan, we could be lenient b'Di'eved, like we are lenient about a Safek Chalutzah. However, there is no source to say that Chachamim decreed that Bi'as Achos Zekukaso forbids to Kehunah. - PF)

(R. Yosi bar R. Ba): R. Chiya taught that one who has Bi'ah with the sister of his Zekukah, he did not disqualify her from Kehunah.

(Mishnah): If each died leaving one brother, each brother does Chalitzah to both.

Question: Everywhere, you say that there is no Chalitzah after Chalitzah, and here you say so?!

Answer: Here [there is no Chalitzah after Chalitzah] regarding Vadai. Here it is a Safek.

(Mishnah): If Reuven left one brother and David left two, Reuven's brother does Chalitzah to both. One of David's brothers does Chalitzah and one does Yibum.

Question: Everywhere, you say that when we do not tell him to do Yibum, we do not tell [even another brother] to do Chalitzah, and here you say so?!

Answer: Here [when we do not tell him to do Yibum, we do not say to do Chalitzah] regarding Vadai. Here it is a Safek.

(Mishnah): If both did Yibum, we do not take from them.