Explanation #1 (Rashi): They fell simultaneously, like R. Yosi ha'Glili, who says that Efshar Letzamtzem (it is possible for things to happen simultaneously).

Explanation #2: The first who fell was Ervah to the second brother who died. She was never Zekukah to him at all.

Observation: We could have answered that R. Shimon address Rabanan according to their opinion.

Implied question: Below (51b), R. Shimon holds that Ma'amar totally acquires!

Answer: [It totally acquires] so that Bi'ah [of a minor above nine years old] and Ma'amar after it do not take effect, but it is not as if he did Yibum.

Observation: This proves like R. Tam, that R. Oshaya is not Rav Oshaya.

Note: Tosfos (Eruvin 68a DH Rav) explains that here, R. Oshaya challenges what Rav Oshaya taught above on 18b [that R. Shimon permits even a brother born before Yibum, i.e. because Zikah is as if Yibum was done. We did not say that he retracted!] Our Tosfos cites Rav Oshaya here. Presumably, his text on 18b said R. Oshaya; also on 9b, our text attributes that teaching to R. Oshaya.

Explanation (Rashi): R. Shimon holds that it is a Safek whether or not a Ma'amar acquires. When he does Yibum or Chalitzah to the second, in any case the one who received the Ma'amar is exempted.

Source #1: Rashi did not want to say that R. Shimon holds that it acquires mid'Rabanan, for if so, he could not do Yibum with the second, for she is Tzaras Eshes Achiv she'Lo Hayah b'Olamo [mid'Rabanan, i.e.] through a Ma'amar.

Rather, surely he holds that it totally acquires, or it does not acquire at all.

Question: Still, she is Tzaras [Ervah] through Zikah!

Answer: R. Shimon holds that Ein Zikah.

Rejection of source (Ri): Even if he held that Yesh Zikah and a Ma'amar acquires a little mid'Rabanan, he could say that Zikah and a Ma'amar forbid only when if she were married, she would forbid;

Here, if she were married, R. Shimon would permit her herself, for he permits [Yibum] when the brother was born after another brother did Yibum.

Affirmation: Rashi's explanation is correct. R. Shimon holds that if Ma'amar does not acquire, it does not acquire even mid'Rabanan.

Source #2: He said above "if the second's Ma'amar is not a Ma'amar, he has Bi'ah with the first brother's wife." This connotes it does not acquire at all.

Question: Why don't we derive from here a refutation of R. Oshaya?

According to him, Zikah is like a Nesu'ah. Why isn't the Tzarah exempted?

Answer: This is mid'Rabanan, like he said above, that it is a decree due to two Yevamos falling from two houses, [lest] one will do Chalitzah, and the other [will think that she] is exempt without anything.

Implied question: This is obvious. There was no need to teach it at all!

Answer (Ri): It was taught only to teach that this Tana mixes and teaches cases together;

Therefore, we are able to deduce that R. Shimon does not argue when he was born and afterwards his brother did Yibum, since he taught them together the case when he did Yibum and died and then a brother was born, which R. Shimon agrees about.

If R. Shimon argued also when he was born and afterwards the brother did Yibum, it should have been taught together with the Seifa, and not with the Reisha.

Support: This explains why we do not challenge R. Oshaya from the Mishnah, which did not teach the cases together. One may deduce [only] from the Beraisa, like I explained.

Explanation #1 (Rashi): This refers to all the cases.

Rejection: Surely R. Shimon does not argue about when he did Yibum, died, and then a brother was born!

Explanation #2: Rather, it refers to the last two cases.

Implied question: Ma'amar is only mid'Rabanan! (How do we learn from Torah laws?)

Answer: Even so, one holds that it is reasonable that Chachamim made it like Bi'ah, and one holds that it is reasonable that Chachamim made it like Kidushin.

Support: We find like this in Bava Metzi'a (11a). We say that one opinion learns [that a minor's Chatzer can acquire for him] a Metzi'ah (a Hefker object) from Get, even though Zechiyah for a minor is only mid'Rabanan, like it says in Gitin (61a).