Implied question: We should be concerned lest if here we permit Yibum to the first after the second did Chalitzah, in general people will come to do oppositely!

Answer: This is not a big concern, lest people permit the second, who was not permitted initially, just because we permitted the first, who was [initially] permitted.

However, if they come to ask from the beginning, we decree, since these Yevamos themselves can come to ruin.

Explanation: We answered that "the Mishnah says they do not do Yibum", i.e. there is no case of doing Yibum at all, even if he already did Chalitzah to the second.

Explanation (cont.): We asked "he should have said [that we forbid Yibum] lest he (the other brother) die [and there is Bitul Mitzvas Yevamin], and establish the Mishnah like the opinion that Ein Zikah!

According to him, surely even if he already did Chalitzah to the second, and for these Yevamos there is no concern [of switching the order], he (the other Yavam) may not do Yibum to the first, for if we permit Yibum here after Chalitzah, in general people will come to do Yibum first;

It does not cross their mind the reason lest by the time one does Yibum, the other will die, for this is not a [commonly] known reason.

Explanation #2 (Ri): We can explain differently. We should say to him "perhaps he will do Chalitzah to the first one first;

And even if he already did Chalitzah by himself to the second, we forbid the first to do Yibum, lest they come to permit the second in general when they already did Chalitzah to the first;

However, when the second died, such a decree does not apply.

He answers that the Mishnah says "they do not do Yibum", i.e. there is no case of doing Yibum at all, even if the second died.

Explanation (cont.): He asks that it is a decree lest [the other Yavam] die. Therefore the Mishnah forbids, even if the second Yevamah died!

If we would permit Yibum when there are two Yevamim after the second Yevamah died, people will come to permit without death.

However, R. Yochanan discusses when there is only one Yavam. When there is only one Yavam, they will not permit without death, for that would immediately be Mevatel Mitzvas Yevamin;

However, when there are two Yevamim, he is not immediately Mevatel Mitzvas Yevamin [through doing Yibum]. It is only a decree lest the other die. People will come to err and do Yibum.

Retraction: The Ri retracted from this Perush, because also when there is one Yavam the Mishnah says that they do not do Yibum;

This connotes that there is no case of doing Yibum at all, even if the second died;

It says below that if two of three brothers married sisters, and those two died, they do Chalitzah, but not Yibum.

Explanation #1: This refers to R. Eliezer below (41a). If a Yavam was Mekadesh the sister of his Shomeres Yavam, if the Yevamah dies, he is permitted to his wife. If his wife dies, the Yevamah does Chalitzah, but not Yibum.

Explanation #2 (Rashi): This refers to R. Eliezer below (109a). If a man divorced his wife and remarried her, she may do Yibum. R. Eliezer forbids.

Observation: We conclude like this according to R. Yochanan, but Rav Chisda establishes the Mishnah (Shevu'os 32a), about two witnesses who denied [knowing testimony] at once, like R. Yosi ha'Glili;

R. Yochanan concludes there that it is even like Rabanan. This is like he taught here.

Pesak: In Eruvin (5b), I explained that we hold that b'Yedei Adam, Efshar Letzamtzem;

Also b'Yedei Shamayim, it seems that Rava (Bechoros 60b) holds that Efshar Letzamtzem regarding two animals that left the pen simultaneously as the 10th [and both become Ma'aser Behemah];

In Chulin (70a), regarding whether we follow the majority of a limb, he (Rava) establishes that exactly half of the child (a stillborn Bechor Behemah) left the womb, including the majority of a limb. (If we view it as if the entire limb came out, a majority was born.)

If you would say that Iy Efshar Letzamtzem, what was the question? In any case it must be buried, for perhaps the majority came out!

This is not the place to elaborate, for there, I elaborated.

Question: In any case this is difficult for Rav, who permits even the second [to fall]!

Answer: Rav holds that Ein Zikah. She is forbidden only due to Bitul Mitzvas Yevamin. We decree lest he do Yibum with the other (who is not Ervah to either Yavam) first.

Question: Does he come to teach the opinion of Beis Shamai?! The Gemara asks like this in Shabbos (135a).

Answer: Here, he needs to teach unlike the Beraisa of Aba Sha'ul, who says that Beis Hillel were lenient about this. (If not for R. Eliezer, one might have thought that our Mishnah is like Beis Shamai.)

Question: We know also this from that Mishnah, like we said above (20a)!

Answer: There, it is clear [that she is not Achos Zekukaso], because there is an Ervah who does not do Chalitzah or Yibum;

Here, both of them do Yibum, so nothing proves [that she is not Achos Zekukaso].

Explanation: This is only when they fell to one Yavam;

However, if they fell to two Yevamim, R. Shimon agrees, like the Beraisa says above, "R. Shimon says, [if they did Yibum,], they remain married."