Mishnayos Bechoros Perek 8 Mishnah 4

With regard to two wives of one man, both of whom had not previously given birth, and they gave birth to two males, i.e., each bore one male, and the sons were intermingled, the father gives ten sela coins to the priest even if it is unknown which son was born first, because it is certain that each is firstborn of his mother. In a case where one of them dies within thirty days of birth, if he gave all ten sela coins to one priest, the priest must return five sela to him, because the father was not obligated to redeem the son who then died. And if he gave the redemption payment to two different priests, he cannot reclaim the money from the possession of either priest, as each could claim that the money that he received was for the living child. If one mother gave birth to a male and one gave birth to a female, or if between them they gave birth to two males and one female, and the children were intermingled, the father gives five sela coins to the priest: In the first case because the male might have preceded the female and in the second case because one of the males is certainly firstborn. If the children were two females and a male, or two males and two females, the priest has nothing here, as it is possible the female was born first to each mother. If one of his wives had previously given birth and one had not previously given birth and they gave birth to two males who became intermingled, the father gives five sela coins to the priest, as it is certain that one of them was born to the mother who had not yet given birth. If one of them dies within thirty days of birth the father is exempt from that payment, as it is possible that the one who died was born to the mother who had not yet given birth. In a case of intermingling where the father died and the sons are alive, Rabbi Meir says: If they gave the five sela coins to the priest before they divided their father’s property between them, they gave it, and it remains in the possession of the priest. But if not, they are exempt from giving the redemption payment to the priest. Rabbi Yehuda says: The obligation to redeem the firstborn already took effect on the property of the father. If the wives gave birth to a male and a female the priest has nothing here, as perhaps the female was born to the mother who had not yet given birth.