Yevamos, Chapter Five, Mishnah Five. This mishnah continues the discussion we began in the previous mishnah, in a case where there is one yavam facing two yevamos.
The words of the Mishnah:
"Chalatz v'chalatz" - he performed chalitzah with one yevamah, and afterward performed chalitzah with the second. This second chalitzah is inherently unnecessary, since once one chalitzah has been performed there is no need for another.
"Chalatz v'asah maamar, nasan get u'va'al" - he performed chalitzah with one of them, and with the second he made a maamar, or gave a get, or had relations with her.
"Ba'al u'va'al" - he had relations with one, and afterward had relations with the second.
"Ba'al v'asah maamar, nasan get v'chalatz" - he had relations with one, and with the second he made a maamar, gave a get, or performed chalitzah.
Regarding all of these the Mishnah rules: "ein achar chalitzah k'lum" - nothing has any effect after chalitzah. Although the Mishnah did not spell it out here, the same is true that nothing has any effect after relations. In other words, whether the chalitzah came first or the relations came first, any act done afterward is meaningless.
Why nothing has effect after chalitzah:
Once the chalitzah has been performed, she is released, and no subsequent act has the power to accomplish anything. Our mishnah follows the view of Rabbi Akiva, that kiddushin does not take effect with those forbidden by a negative commandment, and after the chalitzah there is a negative commandment against marrying her:
Maamar: It is ineffective, since kiddushin does not take effect with her.
Get: It certainly accomplishes nothing.
Relations: They are forbidden relations and have no validity whatsoever.
Why nothing has effect after relations:
Once he had relations with her, he has fulfilled the mitzvah of yibum, and the yevamah has become his full wife in every respect. Therefore:
Relations with the second: They accomplish nothing, and are merely forbidden relations.
Maamar with the second: It accomplishes nothing, since a man may not marry his wife's sister.
Chalitzah with the second: It accomplishes nothing, since the zikah already lapsed the moment he had relations.
In both scenarios:
"bein yavam echad lishtei yevamos" - as in the case with which we began: one yavam facing two yevamos.
"bein shnei yevamin livamah achas" - two surviving brothers and only one yevamah, where each of the yevamin performs with her, one after the other, a maamar, a get, or relations. The very same laws apply in that case as well.
In summary: In this mishnah we learned that chalitzah and relations are decisive acts that bring the zikah to an end: after chalitzah the yevamah is released, and after relations she has become his full wife, and from that point on no maamar, get, chalitzah or further relations accomplish anything, whether with one yavam and two yevamos or with two yevamin and one yevamah.