We have already discussed the concept of zika – the almost marital relationship that exists between the yavam and the yevama before they have had the opportunity to fulfill the mitzva of yibum or halitza (see Yevamot 18). The discussion of zika is revived on our daf in the context of the following case:
There were three brothers and two of them were married to two sisters. The married brothers died, and both widows become potential yevamot to the surviving brother. In this case he certainly cannot marry (i.e. perform yibum with) both sisters, a relationship that is forbidden by the Torah. Can he marry one of them?
Rav Huna quotes Rav as requiring halitza for each of them so that they can marry others. If, however, one of them died, then he would be permitted to marry the other one.
Rabbi Yohanan agrees that each of them will require halitza. In the event that one of the sisters dies, however, he distinguishes between them. If the sister who was widowed second passes away, he will be allowed to perform yibum. If the sister who was widowed first dies, he will not be allowed to perform yibum on the remaining sister because at the moment that her husband died, she was forbidden to the yavam, since her sister was already in a situation of zika to the brother, and he could not marry both sisters.
One explanation given by the rishonim to explain why Rav permits even the second yevama to get married (in the event that her sister died) is presented by Rabbeinu Hananel, the Rashba and others. They explain simply that Rav rejects the very concept of zika (see Yevamot 18). In theory, therefore, he should allow the yavam to marry either of the two sisters, even if they both remain alive. What keeps him from doing that is not connected with zika but with a different rule – that a person cannot perform an act that will negate the mitzva of yibum. By performing yibum on one sister, the yavam is simultaneously negating the mitzva of yibum on the other.