Study Guide Kiddushin 51 If one betrothed two women at the same time who are both forbidden to him on account of each other, such as a woman and her sister or a woman and her mother, neither betrothal is effective. Rami bar Hama and Raba each bring a different explanation for this. The basis for Raba's explanation is that if a man cannot marry another one after the other, then if he betroths them simultaneously, the betrothal is not effective. Difficulties are raised against this principle of Raba from three other areas of halakha (tithes on produce, animal tithes, and the breads of the thanksgiving offering, korban toda) where the same situation exists - that if one does an action one after the other, it would not be effective - however, when done simultaneously, it is effective! Rava and Abaye have a disagreement regarding a betrothal between two people in a situation where the couple cannot have relations (as in the case above where each wife is prohibited to the husband on account of the other - i.e. sister of a woman he is betrothed to). Rava holds that it is not effective and Abaye rules that it is. Five mishnayot, including our Mishna, are brought to raise a difficulty with Rava's position, but each difficulty is resolved.