Yoma 65A

הכל מודים שאם נתכפר בשאינה אבודה ⁠— אבודה מתה.

Everyone concedes that if the animal one designated as a sin-offering became lost, and he designated another animal in its stead and then found the first animal, if he gained atonement through the one that was not lost, i.e., the second one, the one that was lost is left to die.

אלא לרב הוה ליה כמפריש שתי חטאות לאחריות, ואמר רבי אושעיא: הפריש שתי חטאות לאחריות ⁠— מתכפר באחת מהם, והשניה תרעה.

However, according to the opinion of Rav, who rules that the remaining goat from the first pair is the one that is sacrificed, that goat was never disqualified, and the extra goat from the second pair was designated to serve the same purpose as the remaining goat. This is like one who designates two sin-offerings as a guarantee, so that if he loses one of them he will still be able to bring the other one. And R' Oshaya said that all agree that if one separated two sin-offerings as a guarantee, he gains atonement through one of them, and the second is left to graze until it becomes unfit. Consequently, even the sin-offering of an individual is not left to die in this case.

כיון דאמר רבא: רב סבר לה כרבי יוסי דאמר מצוה בראשון, מעיקרא כמפריש לאיבוד דמי.

The Talmud answers: Since Rava said that Rav holds in accordance with the opinion of R' Yosei, who said that the mitzva should be performed with the first animal, when one designates the second animal, it is considered from the beginning to be comparable to an offering separated in order to be lost. In that case, if it is the sin-offering of an individual, the second animal is left to die.

תנן, רבי יהודה אומר: תמות. בשלמא לרבי יוחנן דאמר שני שבזוג ראשון ירעה, לרבי יהודה ימות ⁠— מתכפר בשני שבזוג שני.

We learned in the Mishnah that R' Yehuda says: It should be left to die. Granted, according to the opinion of R' Yoḥanan, who said that the second goat of the first pair should be left to graze, according to R' Yehuda, who said it should be left to die, he nonetheless gains atonement with the second goat of the second pair.