1) PERFORMING KIDUSHIN WITH TWO SISTERS SIMULTANEOUSLY
OPINIONS: When a man marries a woman and then attempts to marry her sister, the Kidushin does not take effect because the second woman is prohibited to him as an Ervah of "Achos Ishto" (his wife's sister). The Mishnah teaches that when a man is Mekadesh two sisters at the same time, the Kidushin does not take effect. The Gemara asks, why does it not take effect? Perhaps the Kidushin should take effect because neither woman was classified as an "Achos Ishto" at the time of the Kidushin since no prior act of Kidushin had been performed with the other.
Rava answers with the principle of Rabah: Any two acts which cannot take effect consecutively also cannot take effect simultaneously ("Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino").
The Gemara entertains the possibility that when a man is Mekadesh two sisters at the same time, the Kidushin could take effect. On whom, though, would the Kidushin take effect? Would only one of the sisters become Mekudeshes or would both of them become Mekudeshes? If only one of the sisters would become Mekudeshes while the other would be forbidden as "Achos Ishto," then each woman would be Safek Mekudeshes (doubtfully betrothed) and the man would need to give each one a Get. On the other hand, perhaps both sisters become Mekudeshes since, at the moment of the Kidushin, there is no "Achos Ishto" to prevent the Kidushin with the other from taking effect.
ANSWERS: This question is discussed at length by the Acharonim.
(a) The Gemara teaches that when a person pledges to bring a Korban Todah (which must be accompanied by forty loaves) and he adds that he wants eighty loaves to be sanctified, none of the loaves become sanctified. RASHI explains that this is because of Rabah's principle, "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino." Since it is impossible to sanctify forty additional loaves when one has already sanctified forty loaves for the Korban Todah (i.e. the loaves cannot be sanctified "Zeh Achar Zeh"), one also cannot sanctify eighty loaves together at one time ("b'Vas Achas Eino"). This implies that if "Bas Achas" would work, all eighty loaves would become sanctified. This case proves that "Bas Achas" would enable both actions to take effect, and thus -- in the case of a man who is Mekadesh two sisters simultaneously -- both sisters should become Mekudeshes.
(b) The Gemara asks why Rava needs to explain the Mishnah's ruling in accordance with Rabah's principle of "Kol she'Eino ba'Zeh Achar Zeh...." Why does he not explain simply that the Kidushin with each sister is a Kidushin which cannot be consummated with marital relations ("Kidushin she'Einan Mesurin l'Bi'ah")? Since each woman might be an "Achos Ishto," the man may not live with either one.
The Gemara answers that Rava does not need to explain the Mishnah's ruling based on the principle of "Kol she'Eino ba'Zeh Achar Zeh..." but he chooses to do so in order to explain the view of Rami bar Chama (50b) who attempts to prove the Mishnah's statement from the verse of "v'Ishah El Achosah Lo Sikach" (Vayikra 18:18).
The Gemara questions Rava's explanation from the Reisha of the Mishnah, which states that when a man attempts to be Mekadesh a woman and her daughter at the same time, the Kidushin is invalid. This implies that if he is Mekadesh only one of them (without specifying which one), the Kidushin is valid, even though it is a "Kidushin she'Einan Mesurin l'Bi'ah" (since each woman is a Safek Ervah). The Gemara (51b) answers that the Mishnah means that one who is Mekadesh one of the two related women (without specifying which one) is like one who is Mekadesh both women, and the Kidushin does not take effect at all.
The RASHBA asks that the only way Rava can adequately explain the Mishnah is with the principle of Rabah that "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino." Why, then, does the Gemara say that Rava does not need to explain the Mishnah according to this principle?
The Rashba answers that when one is Mekadesh both related women simultaneously, since each woman is definitely (Vadai) Mekudeshes each one also becomes a Vadai Ervah. In a case in which the Isur Bi'ah is a Vadai Isur (and not just a Safek), everyone agrees that "Kidushin she'Einan Mesurin l'Bi'ah" is not a valid Kidushin.
The Rashba states clearly that when one is Mekadesh two sisters at the same time, the Kidushin takes effect on both of them (and there is not a mere Safek which one is Mekudeshes, according to the Gemara's Havah Amina). (A. KRONENGOLD)
2) TWO ACTS THAT TAKE EFFECT SIMULTANEOUSLY
QUESTION: The Gemara challenges Rabah's principle of "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino" -- any two acts which cannot take effect consecutively also cannot take effect simultaneously -- from the case of "Marbeh b'Ma'aser," in which a person separates more than a tenth of his fruit as Ma'aser. The Tosefta states that his fruits (from which Ma'aser was separated) are permitted, but the fruits of Ma'aser themselves are "Mekulkal" ("corrupt"). The fruits of Ma'aser are prohibited because Ma'aser can take effect only on a tenth of the fruits. Since he separated more than a tenth as Ma'aser, fruits of Tevel are mixed with the fruits of Ma'aser (and he must separate Ma'aser from another source on behalf of the "Mekulkal" fruits).
According to Rabah's principle of "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino," the Ma'aser should not take effect at all and the fruits he separated should not be prohibited. Since one cannot separate a tenth of his fruit as Ma'aser and then separate another tenth ("ba'Zeh Achar Zeh"), he cannot separate two tenths simultaneously ("Afilu b'Vas Achas Eino"). The Tosefta, however, says that one act of separating Ma'aser does take effect, and thus it refutes the principle of "Kol she'Eino ba'Zeh Achar Zeh."
The Gemara here contradicts the opinion of RASHI and the RASHBA (see previous Insight) who maintain that according to the opinion that does not agree with the principle of "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino," both of the acts take effect.
The same question may be asked from the case of Ma'aser Behemah. The Gemara states that the tenth and eleventh animals are mixed together and the Ma'aser animal is not known. This means that the Kedushah of Ma'aser takes effect on only one of the animals, and there is a doubt about which animal it is.
The same question may be asked from the case of the Lachmei Todah, the loaves offered with the Korban Todah. Chizkiyah rules that forty of the loaves are Hekdesh, but not all eighty. This implies that only one act of Hekdesh takes effect, in contradiction to the views of Rashi and the Rashba.
ANSWER: Certain Halachic acts take effect only once. "Ma'aser" means "one tenth," and by definition it is not possible for two Ma'aseros (two tenths) to take effect simultaneously, since the quantity of fruit would be one fifth and not one tenth. Since two Ma'aseros is in direct contradiction to the very meaning of Ma'aser, only one act of Ma'aser can take effect.
The same reasoning applies to Ma'aser Behemah. By definition Ma'aser Behemah means one animal out of ten and not more. Similarly, the Torah specifies an exact number of loaves of Lachmei Todah that are to be offered with the Korban. Hence, one cannot add to that number by sanctifying more than forty loaves, and if he attempts to do so only forty loaves become sanctified. (See PNEI YEHOSHUA and others.)
If this is true, however, why does the Gemara present these cases in the first place? Since the dispute about two acts done "b'Vas Achas" involves the question of whether both acts take effect (as Rashi and the Rashba explain), why does the Gemara mention these cases altogether? In these cases, it is not possible for both acts to take effect together, and thus they should have no bearing on the question of the Gemara at all.
The Acharonim explain that the dispute revolves around how to view two acts that are done simultaneously. When two acts are done at the same time, does neither one exclude or interfere with the other since neither one has taken effect yet, or does one indeed interfere with the other even in such a situation? If both acts take effect at the same time, what exactly takes effect depends on the particular case. In some cases, both acts take effect (like Kidushin). In other cases, only one act takes effect (like Ma'aser). If, on the other hand, both acts can never take effect at the same time ("Afilu b'Vas Achas Eino"), there is no question about how many acts take effect. Since one act always interferes with the other, neither one takes effect. According to this view, when one performs two acts of separating Ma'aser, neither act takes effect at all because the other act of Ma'aser, which is performed at the same time, excludes it from taking effect. (A. KRONENGOLD)