Bechorot 17 - How Many Generations of Firstborn Exemptions?
Previously we learned that when a Jew accepts from an idolater sheep valued at a fixed sum, with the arrangement of sharing in the offspring, the offspring are exempt from the laws of the firstborn. Rav Huna understands this to mean only the next generation, but Rav Yehudah says that the next two generations are exempt, because this is how far the lien extends. They derive their respective views from the analysis of the dissenting opinion of Rabban Shimon ben Gamliel who says that all generations are exempt.
If an ewe gave birth to two male lambs and their heads emerged from the womb simultaneously, then Rabbi Yose HaGlili says that both are firstborn and both are given to a kohen, since the Torah said, " The males belong to God ." However, the Sages disagree and say that it is impossible for two events to happen exactly at the same time. Therefore, one was the firstborn, only we don't know which one, and the kohen gets one animal. Rabbi Yose then says that this Torah phrase is exactly the source teaching that simultaneous events occur. And the Sages? The say that "males" just means all males in general.
Art: Richard Ansdell - A Ewe with Lambs and a Heron Beside a Loch