Bava Batra 57A

ההוא שטרא דהוה חתימי עליה בי תרי, שכיב חד מינייהו. אתא אחוה דהאי דקאי, וחד אחרינא, לאסהודי אחתימת ידיה דאידך.

The Talmud relates: There was a certain document that had the signatures of two individuals on it. One of the signatory witnesses died, and when the court sought to ratify the document, which requires either that the witnesses personally attest to the validity of their signatures or that two other witnesses attest to the validity of the signatures, the brother of the witness who remained alive and one other individual came to testify with regard to the signature of the other, deceased, witness, while the living witness attested to his own signature.

סבר רבינא למימר: היינו מתניתין ⁠– שלשה אחין, ואחד מצטרף עמהן.

Ravina thought to say that this case is the same as the halakha in the Mishnah, which states that if testimony was given by 3 brothers, each of whom testify about one year, and another, unrelated individual joined with each of the brothers as the second witness, these are considered to be 3 distinct testimonies. Similarly, in this case, one brother attests to his own signature, while the other brother attests to the signature of the deceased witness.

אמר ליה רב אשי: מי דמי?! התם לא נפיק נכי ריבעא דממונא אפומא דאחי, הכא נפיק נכי ריבעא דממונא אפומא דאחי.

Rav Ashi said to him: Are these cases comparable? There, the property less 1/4th, i.e., 3/4ths of the property in question, is not removed from the possession of the prior owner based upon the mouth, i.e., the testimony, of brothers. Each brother provides only half of the testimony for each year about which he testifies. Here, the property less 1/4th is removed from the possession of the debtor detailed in the document based upon the mouth of brothers. One brother attests to his own signature, which constitutes half of the testimony, while the other signature is authenticated by the testimony of the other brother and another individual. It follows that 3/4ths of the testimony is given by brothers, and it is therefore invalid.