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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