Zevachim 6A

ושותפין לא מצו ממירין. אלא אי אמרת לא קניא להו ⁠– אמורי נמי לימרו!

and partners cannot effect substitution of other animals for their offering. But if you say that it is not acquired by them, and the animal is the property solely of the deceased father, let them also effect substitution on his behalf, as heirs are able to effect substitution for their deceased parents’ offerings.

שאני התם, דאמר קרא: ״אם המר ימיר״⁠ – לרבות את היורש; אחד ממיר ואין שנים ממירין.

The Talmud answers: There it is different, as although the verse states: “If he should at all change [hamer yamir] animal for animal” (Leviticus 27:10), the superfluous word hamer serving to include the heir as one who is able to effect substitution, nevertheless the subject’s singular form teaches that only one heir can effect substitution, but two heirs cannot effect substitution.

מתקיף לה רב יעקב מנהר פקוד: אלא מעתה, גבי מעשר דכתיב: ״ואם גאל יגאל״ לרבות את היורש ⁠– הכי נמי אחד גואל ואין שנים גואלין?!

Rav Ya’akov of Nehar Pekod objects to this derivation: If that is so, one should say the same with regard to the redemption of the second tithe, as it is written: “And if a man will redeem [gaol yigal] any of his tithe, he should add to it the fifth part thereof” (Leviticus 27:31), with the superfluous word gaol serving to include the heir as one who must add the 1/5th. So too, it should be derived from the verb’s singular form that if one heir redeems the tithe, he must add 1/5th of its value, but if two heirs redeem it, they do not need to add 1/5th. In fact, the halakha is that partners must also add 1/5th.

שאני מעשר, דגבי אבוהון נמי איתיה בשותפות.

The Talmud answers: Redemption of the tithe is different, as the father had the ability to redeem the tithe even in partnership with another when alive. Therefore, the heirs can do so as well. By contrast, substitution of an offering cannot be effected by partners.