Related for Yevamot 2:7
שְׁנַיִם שֶׁקִּדְּשׁוּ שְׁתֵּי אֲחָיוֹת, זֶה אֵינוֹ יוֹדֵעַ אֵיזוֹ קִדֵּשׁ, וְזֶה אֵינוֹ יוֹדֵעַ אֵיזוֹ קִדֵּשׁ, זֶה נוֹתֵן שְׁנֵי גִטִּין, וְזֶה נוֹתֵן שְׁנֵי גִטִּין. מֵתוּ, לָזֶה אָח, וְלָזֶה אָח, זֶה חוֹלֵץ לִשְׁתֵּיהֶן, וְזֶה חוֹלֵץ לִשְׁתֵּיהֶן. לָזֶה אֶחָד וְלָזֶה שְׁנַיִם, הַיָּחִיד חוֹלֵץ לִשְׁתֵּיהֶן, וְהַשְּׁנַיִם, אֶחָד חוֹלֵץ וְאֶחָד מְיַבֵּם, קָדְמוּ וְכָנְסוּ, אֵין מוֹצִיאִין מִיָּדָם. לָזֶה שְׁנַיִם וְלָזֶה שְׁנַיִם, אָחִיו שֶׁל זֶה חוֹלֵץ לְאַחַת, וְאָחִיו שֶׁל זֶה חוֹלֵץ לְאַחַת, אָחִיו שֶׁל זֶה מְיַבֵּם חֲלוּצָתוֹ שֶׁל זֶה, וְאָחִיו שֶׁל זֶה מְיַבֵּם חֲלוּצָתוֹ שֶׁל זֶה. קָדְמוּ שְׁנַיִם וְחָלְצוּ, לֹא יְיַבְּמוּ הַשְּׁנַיִם, אֶלָּא אֶחָד חוֹלֵץ וְאֶחָד מְיַבֵּם. קָדְמוּ וְכָנְסוּ, אֵין מוֹצִיאִין מִיָּדָם:
If two [who were not brothers] betrothed two sisters, each not knowing which he betrothed, each gives two gittin. If they died, and each had a brother, each gives chalitzah to both, [each being forbidden to perform yibum because of the possibility of "the sister of his linked one"]. If one had one brother and the other two, the one brother gives chalitzah to both, [yibum being impossible both before the chalitzah of the second woman, by reason of "the sister of his linked one," and after, by reason of "the sister of his chalutzah." Therefore, since yibum is forbidden, he first gives chalitzah to both, to free the one who is his yevamah to others]; and the two brothers — one gives chalitzah [to dissolve the linkage from his brother, if this is his yevamah], and the second takes (the other) in yibum. [The sister is permitted to the second in any case. If she is his yevamah, all well and good; and if she is the sister of his yevamah, the linkage of the latter was dissolved by his brother's chalitzah.] If they were beforehand and married them, they are not taken from them. If each had two brothers, the brother of one gives chalitzah to one, and the brother of the other gives chalitzah to the other. The latter brother takes the chalutzah of the former in yibum, and the former brother takes the chalutzah of the latter in yibum. [For, in any case, if the one he takes is his yevamah, he has taken her correctly, his brother having given chalitzah not to her, but to her sister, who was not his yevamah, so that the chalitzah is of no import. And if the one he takes is not his yevamah, he has properly wed an unattached woman. For "the sister of his linked one" does not obtain here, his brother having given chalitzah to her sister, his yevamah, so that the linkage is dissolved.] If two [brothers of (one of the deceased)] were beforehand, and gave chalitzah [to both of them, (each) not knowing which was his yevamah], the two [other brothers] may not take them [respectively] in yibum [for it may be that the first would be taking the sister of his linked one], but one gives chalitzah [first to one of them], and the other takes [the second] in yibum. [For, in either case, if she is his yevamah, all well and good; and if not, he will not have violated "the sister of his linked one," his brother having given chalitzah to his yevamah. And she (the latter) will not be considered "a yevamah to the marketplace," having received chalitzah from her husband's brother.] And if they [the two other brothers] were beforehand, and married them [after the chalitzah of the first two, not having consulted beth-din], they are not taken from them [as explained before. For there obtains only the possibility of "the sister of his linked one" in the marriage of the first. And it may be that he had actually, and properly, married his yevamah, so that the other brother, too, would properly have married an unattached woman. And even if the opposite were the case, and the first had violated "the sister of his linked one," when the second came and took the other in yibum, her linkage would have been (retroactively) dissolved and his (the first brother's) wife would be permitted to him, though he had transgressed in taking her.]