On the last daf we discussed a case where there were three brothers, two of whom were married to two sisters. The married brothers died, and both widows become potential yevamot to the surviving brother. The Mishna on our daf teaches that although the Tanna Kamma requires that both widows receive halitza, there is an opinion – Rabbi Shimon – that rules that these two sisters are permitted to marry whoever they want without a need for halitza (the brother certainly cannot marry both sisters, a relationship that is forbidden by the Torah).
The Gemara on our daf suggests that Rabbi Shimon does not require halitza for these sisters because of his reading of the passage in (18:18) that forbids marrying two sisters. He understands that the pasuk teaches that when sisters somehow gain the status of tzarot (rival wives), as they do in our case, neither of them will be permitted. The Talmud Yerushalmi presents an alternative reading of Rabbi Shimon, arguing that he did not intend to forbid yibum with both of the widowed sisters, rather only with the one whose husband died second. Rabbi Oshaya explains that Rabbi Shimon views zika (see Yevamot 18) as being the equivalent of actual marriage. Thus, when the second brother passes away, his widow is freed from any need of halitza, according to Rabbi Shimon, because the yavam is already “married” to the widow of the first brother who died.
Tosafot point out that this would be true only in the case where there is just one potential yavam. If there were more surviving brothers, then Rabbi Shimon also admits that we would allow the brothers to remain married to the widowed sisters, if each of them performed yibum with a different one. Clearly, according to Rabbi Shimon we need to distinguish between the case of a single yavam where zika is tantamount to marriage, and more than one potential yavam, where the zika relationship remains something less than that.