QUESTION: Rebbi Shimon maintains "Ein Isur Chal Al Isur" -- once an object is prohibited with one Isur, it cannot become prohibited again with another Isur. Accordingly, when two brothers were married to two sisters and one brother died, if the surviving brother does Yibum with his brother's wife (to whom he is prohibited because of "Achos Ishto"), he is liable for transgressing only one Isur: If the first brother was married first the Isur of "Eshes Ach" took effect, and when the second brother marries the sister of his brother's wife the Isur of "Achos Ishto" cannot take effect. If, however, the second brother married first and his wife's sister became forbidden to him as "Achos Ishto," when his brother marries the second sister the Isur of "Eshes Ach" cannot take effect.

Beis Shamai makes a similar statement earlier (13b), where he rules that one is liable only for his transgression of the Isur which took effect first. Beis Shamai concludes that the Tzarah of an Ervah is permitted to do Yibum: if the deceased brother had married the other wife before she became an Ervah to the second brother, the other wife is only an "Eshes Ach" to the brother, in which case the Tzarah is not the Tzarah of an Ervah but rather the Tzarah of merely an "Eshes Ach" (and thus she may do Yibum). If the deceased brother had married the other wife after she became forbidden as "Achos Ishto" to the second brother, the Tzarah is only the Tzarah of an "Achos Ishto," and since an "Achos Ishto" does not fall to Yibum at all (since only a woman who is an "Eshes Ach" falls to Yibum) she does not preclude the Tzarah from doing Yibum.

Does this imply that Rebbi Shimon (and every other Tana who maintains "Ein Isur Chal Al Isur") agrees with Beis Shamai that the Tzarah of an Ervah may perform Yibum? (TOSFOS YESHANIM 13b)

ANSWER: Although Beis Shamai associates the two issues and says that the allowance for a Tzaras Ervah to do Yibum depends on "Ein Isur Chal Al Isur," the other Tana'im who maintain "Ein Isur Chal Al Isur" assert that the two issues are not related. A Tzaras Ervah is prohibited from doing Yibum even if "Ein Isur Chal Al Isur."

The reason for this is that the Gemara says that although only one Isur actually takes effect, the other Isur is "Mitla Tali v'Kai" -- it is suspended and waiting for an opportunity to take effect. As soon as the first Isur is removed, the second Isur takes effect. Rebbi Shimon (and those who rule, like Beis Hillel, that a Tzarah of an Ervah is prohibited from doing Yibum) maintain that the fact that the Isur is suspended and waiting to take effect is enough to give the woman the status of both an "Eshes Ach" and an "Achos Ishto" as far as Yibum is concerned. In a case in which the Isur of "Eshes Ach" took effect first, the Yevamah is considered forbidden as an Ervah because of the Isur of "Achos Ishto" which is suspended and waiting to take effect. In a case in which the Isur of "Achos Ishto" took effect first, she still falls to Yibum because she is considered an "Eshes Ach" due to the Isur of "Eshes Ach" that is suspended and waiting to take effect. That potential Isur is enough to make her fall to Yibum.