Yevamos 27

We say that the proof that a Get is no better than Maamar and Maamar is no better than Get is from the Shita of R' Gamliel. Although he holds that once a Get is given to one Yevama then a second Get to a different one does not take effect (and doesn't disqualify her to marry the brother's relatives.) The same thing applies with giving Maamar after a Maamar. However, he allows a Maamar to take effect after a Get and a Get after a Maamar. So we must say they're even.

Tosfos asks: This last Halacha is true even to the Rabanan (that hold that there is Get after Get and Maamar after Maamar), so why does the Gemara only bring a proof from R' Gamlilel?

Tosfos answers the Rabanan that hold a second Get take effect after the first Get despite that it must be weaker (since the first Get was the one that "divorced" her and the second is "divorcing" someone who is already "divorced".) Therefore, even if Maamar or Get is stronger than the other, it will still take effect.

However, R' Gamliel who holds the second Get or Maamar doesn't take effect after the first, because he holds that a weaker transaction can't take effect on a stronger transaction. Therefore, since a Get takes effect on a Maamar and a Maamar takes effect on Get, we must conclude that one is not weaker than the other and therefore they're even.

Rabbi Chaim Smulowitz 

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Yevamos 26

According to those that don't hold of Zika, the problem why two brothers can't do Yibum to two sisters (one brother to each sister) is: we're afraid after one does Yibum to one sister the second brother would die. Therefore the Mitzvah of Yibum will fall off the second sister (since now she's an Ervah, your wife's sister.) So better do Chalitzah on both. Even if the second brother dies before he does Chalitza to the second sister, the first brother can still do the Mitzvah of Chalitza. However if there are three brothers we don't need to be concerned that two of them will die, and one may do Yibum on one of them.

Tosfos asks: here it seems that one needs to be concerned about one person dying and not two.  However, in the beginning of Yuma, the Mishna implies that if you need to be concerned of the death of one you need to be concerned of the death of two. The Chachumim say that though a Kohain Gadol needs to have a wife for Yom Kippur, he doesn't need to marry a second wife in the event that his first wife dies. If you need to be concerned, then two wives wouldn't be enough since you'll need to worry that both of them will die.

Tosfos answers : Over there refers to the concern that the person will die in a small space of time, that day of Yom Kipur. If you're going to be that paranoid, then you'll have to worry about two deaths on that day. Over here, since it might be weeks or months between the first brothers Yibum and the second brothers then it's not so far out  that the second brother might die. Therefore it's a big concern that one might die, however, even over that space of time, we don't need to be concerned that both brothers will die.

Rabbi Chaim Smulowitz
Tosfos Project

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