Yevamos, chapter five, mishnah 4. At the end of the previous mishnah we learned that the bond created by a get, a ma'amar and the like applies to one yevamah and one yavam. Our mishnah comes to clarify how these laws work when we are dealing with two yevamos and one yavam - two different women and one surviving brother.
Cases in which the ma'amar came first:
"Asah ma'amar b'zu u'ma'amar b'zu" - he made a ma'amar with this one and a ma'amar with that one: "they require two gittin and a chalitzah." A get is needed for each woman, corresponding to each ma'amar, since a ma'amar is a rabbinic form of acquisition and therefore requires a get; and only one chalitzah, performed with one of them, in order to release them both from the zikah. This ruling follows the opinion of the Sages in mishnah 1, who hold that a ma'amar can take effect after a ma'amar, and not that of Rabban Gamliel, who holds that a ma'amar cannot take effect after a ma'amar.
"Ma'amar b'zu v'get la'zu" - a ma'amar with this one and a get to that one: "she requires a get and a chalitzah." A get for the woman with whom he made the ma'amar, and a chalitzah with one of them to release the zikah.
"Ma'amar b'zu u'vi'ah b'zu" - a ma'amar with this one and cohabitation with that one: "they require two gittin and a chalitzah." The cohabitation with the second woman is an invalid cohabitation, for once he made a ma'amar with the first she became partially acquired to him and the second was rendered unfit, so her cohabitation accomplishes nothing. Nevertheless two gittin are required: one corresponding to the ma'amar made with the first, and one corresponding to the cohabitation with the second; and in addition a chalitzah with one of them, to release the zikah.
"Ma'amar b'zu v'chalatz la'zu" - a ma'amar with this one and chalitzah with that one: the first woman, with whom the ma'amar was made, requires a get, but no further chalitzah is needed. The chalitzah was effective and released them both from the zikah, yet the ma'amar still requires a get.
Cases in which the get came first:
"Get la'zu v'get la'zu" - a get to this one and a get to that one: they require only a chalitzah. The gittin themselves do not create any requirement afterward, even though they disqualify the yevamah from yibum; and since in any case the intention is to perform chalitzah, one chalitzah suffices and they are both released.
"Get la'zu u'vi'ah la'zu" - a get to this one and cohabitation with that one: "she requires a get and a chalitzah." The cohabitation with the second is invalid, because a get had already been given to her fellow wife. Therefore a get is required for the one with whom he cohabited, and a chalitzah with one of them in order to be released from the zikah.
"Get la'zu u'ma'amar b'zu" - a get to this one and a ma'amar with that one: "she requires a get and a chalitzah." A get for the one with whom the ma'amar was made, while the other does not need one; and a chalitzah with one of the two to release the zikah.
"Get la'zu v'chalitzah la'zu" - a get to this one and chalitzah with that one: nothing further is required, since "nothing takes effect after chalitzah."
"Ein achar chalitzah klum" - nothing takes effect after chalitzah:
Chalitzah has a finality to it, and nothing can take effect after it. The Gemara explains that this wording follows the opinion of Rabbi Akiva, who holds that kiddushin do not take effect with those forbidden by a negative prohibition (chayavei lavin). Once chalitzah has been performed, even if he were afterward to betroth the yevamah the kiddushin would be ineffective, because "since he did not build, he may no longer build" - since he did not build up his brother's house through yibum but rather performed chalitzah, he may no longer build it and cannot marry her. It emerges that betrothing her falls into the category of chayavei lavin, and according to Rabbi Akiva such kiddushin do not take effect.
According to the Sages, however, who hold that a child born of chayavei lavin is not a mamzer and that kiddushin do take effect with them, if he were to betroth her after the chalitzah the kiddushin would take effect and would require a get. And since our mishnah rules that after the chalitzah nothing further is required, even if he performed kiddushin afterward, this proves that it accords with the opinion of Rabbi Akiva, that kiddushin do not take effect with chayavei lavin.
In summary: in this mishnah we learned the laws of the various combinations in a case of two yevamos and one yavam. When the ma'amar came first, each ma'amar requires a get, and cohabitation performed afterward is invalid and likewise requires a get, and in all these cases one chalitzah is needed to release them both from the zikah, unless a chalitzah has already been performed. When the get came first, the get itself creates no requirement after it but does disqualify the woman from yibum, and whatever is done after it (cohabitation or a ma'amar) requires a get and a chalitzah. Finally we examined the finality of chalitzah, and how the wording "nothing takes effect after chalitzah" fits the opinion of Rabbi Akiva, that kiddushin do not take effect with chayavei lavin.