Reference for Bekhorot 2:6
רָחֵל שֶׁלֹּא בִכְּרָה וְיָלְדָה שְׁנֵי זְכָרִים וְיָצְאוּ שְׁנֵי רָאשֵׁיהֶן כְּאֶחָד, רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר, שְׁנֵיהֶן לַכֹּהֵן. שֶׁנֶּאֱמַר (שמות יג), הַזְּכָרִים לַה'. וַחֲכָמִים אוֹמְרִים, אִי אֶפְשָׁר, אֶלָּא אֶחָד לוֹ וְאֶחָד לַכֹּהֵן. רַבִּי טַרְפוֹן אוֹמֵר, הַכֹּהֵן בּוֹרֵר לוֹ אֶת הַיָפֶה. רַבִּי עֲקִיבָא אוֹמֵר, מְשַׁמְּנִים בֵּינֵיהֶן, וְהַשֵּׁנִי יִרְעֶה עַד שֶׁיִּסְתָּאֵב. וְחַיָּב בַּמַּתָּנוֹת. רַבִּי יוֹסֵי פּוֹטֵר. מֵת אֶחָד מֵהֶן, רַבִּי טַרְפוֹן אוֹמֵר, יַחֲלוֹקוּ. רַבִּי עֲקִיבָא אוֹמֵר, הַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה. זָכָר וּנְקֵבָה, אֵין כָּאן לַכֹּהֵן כְּלוּם:
If a ewe that has never given birth before bear to two male lambs, and both their heads came out at once, Rabbi Yossi Haglili says: Both go to the priest, as it says (Exodus 13:12): "The males shall be for God." The Sages say: This is not possible. Rather one goes to [the owner], and the other goes to the priest. Rabbi Tarfon says: The priest chooses the better one. Rabbi 'Akiva says: They compromise. The second one goes out to graze until it develops a blemish, and it is obligated regarding the [priestly] gifts. Rabbi Yossi exempts it. If one of them dies, Rabbi Tarfon says: They divide [the value of the remaining one]. Rabbi 'Akiva says: One who comes to extract from one's friend has the burden of proof. [If she gives birth to] a male and a female, the priest receives nothing.