Yevamot, Chapter Thirteen, Mishnah Nine
This mishnah deals with a case where a man has intercourse with both of his brothers widows, or he has intercourse with one and his brother has intercourse with the other.
1) If a man who was married to two orphans who were minors died, and the yavam had intercourse with one, and then he also had intercourse with the other, or his [the yavams] brother had intercourse with the other, he has not thereby disqualified the first [for him];
2) And the same is true with regard to two deaf women.
3) [If one was] a minor and the other deaf, and the yavam had intercourse with the minor and then he had intercourse with the deaf widow, or a brother of his had intercourse with the deaf widow, he has not disqualified the minor [for him].
4) If the yavam had intercourse with the deaf widow and then he also had intercourse with the minor, or a brother of his had intercourse with the minor, he has disqualified the deaf widow [for him].
Section one: In this case the man had intercourse with both minor wives, or he had intercourse with one and his brother had intercourse with the other. In any case, the first minor widow is still permitted to the first yavam. This is because there is a doubt whether or not yibbum is truly effective in acquiring a minor as a wife. If it is effective, than the first act of yibbum makes her fully his wife, and the second act of intercourse is illicit but does not effect the first wifes status. If it is ineffective, then he didnt need to have yibbum with either minor widow, because they were not biblically married to his brother. In either case, he may remain married to the first one. He cannot stay with the second lest intercourse is effective for acquiring a minor and he has already acquired her rival wife.
Section two: The same rule is true where both wives were deaf-mutes.
Section three: In this case one widow was a deaf-mute and the other a minor. If the yavam had relations first with the minor and then with the deaf-mute, or the yavams brother had relations with the deaf-mute, the minor may remain married to the first yavam. This is for the same reasons that we explained above: if yibbum is effective in acquiring the minor, then she is married to the yavam and the intercourse with the deaf-mute wife is licentious, but does not effect the first wifes status. If it is ineffective, then she was never liable for yibbum with him, because she was not married to his brother.
Section four: In this case, the yavam first has relations with the deaf-mute and then he or his brother has relations with the minor widow. In this case the deaf-mute wife becomes forbidden to him. This is because the acquisition of the deaf-mute is certainly valid, but is not a complete acquisition. The acquisition of the minor is doubtful, as we explained above, but if it does acquire it does so fully. If the acquisition of the minor was fully valid, then it disqualifies the deaf-mute, whose acquisition was only partially valid.