(Mishnah): Reuven, Shimon and Levi (brothers) were married to unrelated women. Reuven died. Shimon gave a Ma'amar to the Yevamah, and then he died. Both Yevamos do Chalitzah, but not Yibum;

"U'Mes Echad Mehem Yevamah Yavo Aleha" - this is when she has Zikah from one brother, and not from two.

R. Shimon says, Levi may do Yibum with whichever he wants, and Chalitzah to the other.

(Gemara) Question: If a Shomeres Yavam fell in front of several Yevamim, does she not do Yibum [because she has Zikah from multiple brothers]?!

Answer (R. Chiyah - Beraisa): The wife of one man does Yibum, but not the wife of two Mesim.

Question: Granted, he must do Chalitzah to Ba'alas ha'Ma'amar, because the wife of one Mes does Yibum, but not the wife of two Mesim. Why can the second [Yevamah] do Yibum?

Answer (R. Lazar): Our Mishnah is R. Meir. R. Meir says that any woman that you may not do Yibum with her, you may not do Yibum with her Tzarah.

R. Yochanan: If R. Lazar (my Talmid) said this, he heard it from me and said it!

Question (R. Yosi - Mishnah): If a boy of nine years and a day had Bi'ah with his Yevamah (Sarah) and married another woman (Leah) after he became an adult [and then died] - if he did not have Bi'ah with Sarah in adulthood, Leah may do Chalitzah or Yibum. Sarah does Chalitzah, but not Yibum;

Here you say that [both women] do Chalitzah, and there you say that [one] may do Yibum?! (Bi'ah of a minor boy above nine is like Ma'amar of an adult. Also there, R. Shimon permits Yibum with either. If Chachamim of R. Shimon hold like R. Meir, also there they should forbid Yibum with the Tzarah!)

Note: I did not understand the other Meforshim, who say that R. Yosi supports R. Yochanan. I gave my own Perush. (PF)

Answer: Here [the first Tana] says that [both] do Chalitzah - R. Meir argues with R. Shimon. There [the first Tana] says that [the Tzarah] may do Yibum - R. Shimon argues with Rabanan [who argue with R. Meir. They do not say that any woman that you may not do Yibum with her, you may not do Yibum with her Tzarah.]

(R. Avin citing R. Bisna citing R. Acha): [R. Shimon holds that Ma'amar acquires totally, or not at all. He permits Yibum with either, but] he must do Yibum first, and then Chalitzah. If he did Chalitzah first, perhaps Ma'amar acquired, and [the other] was exempted through Chalitzah of her Tzarah!