Before us is Mishnah 9 in the fourteenth chapter of Maseches Yevamos, which deals with two brothers - one deaf and one of sound senses - who are married to two women, one deaf and one of sound senses. The Mishnah discusses what the law of the woman is when one of the brothers dies, and which of the brothers may divorce his yevamah after he has married her.
The case before us:
"Shnei achim, echad cheresh v'echad pike'ach" - one brother is deaf and one brother is of sound senses.
"Nesu'im lishtei nachriyos" - two women who are not related to one another, that is, they are not sisters.
"Achas chereshes v'achas pikachas" - the deaf woman is married to the deaf brother, and the woman of sound senses is married to the brother of sound senses.
The deaf husband of the deaf woman died:
What should the brother of sound senses, the husband of the woman of sound senses, do? Chalitzah is not an option, since the yevamah - his brother's widow - is deaf, and she is unable to perform chalitzah. Therefore "kones" - he must perform yibum with her. And the Mishnah adds: "Im ratzah l'hotzi - yotzi" - if he wishes to divorce her with a get, he is permitted to do so, and his get is effective, for he is of sound senses.
The husband of sound senses of the woman of sound senses died:
What should the deaf brother, the husband of the deaf woman, do? Here too chalitzah is not an option, because he is deaf. Therefore "kones" - he must perform yibum with her. However, "ein motzi l'olam" - he may never divorce her, because his get is only of rabbinic force, and it does not have the power to dissolve a bond that is of Torah force, which came about through his brother, whose kiddushin and nisuin were valid by Torah law.
In summary: in both of the cases before us chalitzah is impossible - in the first case because of the deaf yevamah, and in the second case because of the deaf yavam - and therefore in both of them the law is "kones". The difference between them lies in the power of divorce: the brother of sound senses, whose get is of Torah force, may divorce her if he wishes; whereas the deaf brother, whose get is only of rabbinic force, cannot use it to dissolve a bond created through the kiddushin and nisuin of his brother of sound senses, and therefore he may never divorce her.