Yevamos, Chapter 2, Mishnah 7. This mishnah is very similar to the previous one: the very same case, only doubled - the number of people involved is doubled, and the results are very similar, though slightly different.
The case in the mishnah:
"Shenayim shekiddshu shtei achayos" - two men who are unrelated to each other, and each of them betrothed one of two sisters. "Zeh eino yode'a eizo kiddesh v'zeh eino yode'a eizo kiddesh" - each of them knows that he betrothed one of the sisters, but they do not know who betrothed whom.
"Zeh nosen shnei gittin v'zeh nosen shnei gittin" - each of them gives a get to both sisters, and in this way they are properly divorced. But as we mentioned in the previous mishnah, neither man can go through with the marriage to either woman, since it is possible that the second one is his wife's sister.
"Meisu, lazeh ach v'lazeh ach":
If both husbands died, and each of them had only one brother, the law is as in the previous mishnah: "Zeh choleitz lishtayhen v'zeh choleitz lishtayhen" - the brother of the one performs chalitzah with both sisters, and so does the brother of the other. Neither of them can perform yibum, since it is possible that this is his brother's wife; and they cannot even marry them after the chalitzah, since it is possible that she is the woman who received chalitzah from his brother.
"Lazeh echad v'lazeh shenayim":
If one of the deceased had one brother and the other had two brothers - "hayachid choleitz lishtayhen", as explained above. "V'hashenayim - echad choleitz v'echad meyabem", as we saw in the previous mishnah: when there are two brothers, one performs chalitzah and the other performs yibum. For once one of the sisters has received chalitzah, the zikah has been removed: if she is the correct yevamah - the zikah has been removed, and if the yevamah is the other one - then he is performing yibum with her properly.
However, this applies only after the single brother has performed chalitzah with both of them, since each of them may be the yevamah of the other brother who died, and requires chalitzah from him. Therefore this is the order: first the single brother performs chalitzah with both of them, then one of the two brothers performs chalitzah with one of them, and then the second one performs yibum with the other.
And if the two brothers went ahead and both performed yibum, instead of one performing chalitzah and the other yibum - "ein motzi'in miyadam". The women are not taken away from them, and they may remain married to them, as we saw in the previous mishnah: even though they transgressed the possibility of marrying his wife's sister, in any case the zikah has already been removed from both women at this stage.
"Lazeh shenayim v'lazeh shenayim":
If each of the two deceased had two brothers, this is the order of events: "Achiv shel zeh choleitz l'achas, va'achiv shel zeh choleitz l'achas" - a brother from this side performs chalitzah with one of the women, and a brother from that side performs chalitzah with the other. And then "achiv shel zeh meyabem chalutzaso shel zeh, va'achiv shel zeh meyabem chalutzaso shel zeh" - since both of them received chalitzah from one side, that possibility has been eliminated, and yibum is possible.
And if two of the brothers from one side went ahead and performed chalitzah, the two brothers from the other side still cannot both perform yibum with both women. For even though the possibility relating to one of the deceased has already been dealt with through chalitzah, in any case each woman they encounter may be the yevamah who had been betrothed to their brother, and therefore they cannot both perform yibum at the outset, lest they violate the prohibition of his wife's sister.
Rather, one performs chalitzah and the other performs yibum. Here too, however, if they went ahead and both performed yibum - "ein motzi'in miyadam", and the women are not taken away from them, but rather they remain married to them, because the zikah has already been removed.
In summary: in this mishnah we learned the law of two men who betrothed two sisters and it is not known who betrothed whom: each one gives two gittin, and he cannot go through with the marriage. Once they died, when each one has a single brother - both perform chalitzah with both sisters; when one has a single brother and the other has two brothers - the single brother performs chalitzah with both of them, and of the two, one performs chalitzah and one performs yibum; and when each one has two brothers - each brother performs chalitzah with one, and afterward each one performs yibum with the woman who received chalitzah from the other. And in all these cases, if they went ahead and deviated from the proper order and performed yibum - the women are not taken away from them.