Mishnayos Bechoros Perek 8 Mishnah 5

With regard to two women who had not previously given birth, who were married to two different men, and they gave birth to two males and the sons were intermingled, this father gives five sela coins to a priest and that father gives five sela coins to a priest, as each is certainly firstborn to his mother. In a case where one of them dies within thirty days of birth, if the fathers gave all ten sela coins to one priest, the priest must return five sela coins to them. But if they gave the redemption payment to two different priests they 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 the women gave birth to a male and a female and the children became intermingled, the fathers are exempt, as each could claim that he is the father of the female, but the son is obligated to redeem himself, as he is certainly a firstborn. If two females and a male were born, or two females and two males, the priest has nothing here, as it is possible the female was born first to each mother.

שְׁתֵּי נָשִׁים שֶׁל שְׁנֵי אֲנָשִׁים שֶׁלֹּא בִכְּרוּ וְיָלְדוּ שְׁנֵי זְכָרִים, זֶה נוֹתֵן חָמֵשׁ סְלָעִים לַכֹּהֵן, וְזֶה נוֹתֵן חָמֵשׁ סְלָעִים לַכֹּהֵן. מֵת אֶחָד מֵהֶן בְּתוֹךְ שְׁלשִׁים יוֹם, אִם לְכֹהֵן אֶחָד נָתְנוּ, יַחֲזִיר לָהֶן חָמֵשׁ סְלָעִים. אִם לִשְׁנֵי כֹהֲנִים נָתְנוּ, אֵינָן יְכוֹלִין לְהוֹצִיא מִיָּדָם. זָכָר וּנְקֵבָה, הָאָבוֹת פְּטוּרִין, וְהַבֵּן חַיָּב לִפְדּוֹת אֶת עַצְמוֹ. שְׁתֵּי נְקֵבוֹת וְזָכָר אוֹ שְׁנֵי זְכָרִים וּשְׁתֵּי נְקֵבוֹת, אֵין כָּאן לַכֹּהֵן כְּלוּם: