Ketubot 21A

על כתב ידן הם מעידים. לדברי חכמים, על מנה שבשטר הם מעידים.

the witnesses are testifying about their handwriting and authenticating their own signatures. Therefore, if each witness testifies only with regard to his own handwriting, there is only one witness authenticating each signature. According to the Rabbis, the witnesses are testifying about the sum of 100 dinars that is in the document and are not authenticating the signatures at all. Therefore, the testimony of the two witnesses who signed the document is sufficient to ratify the document.

פשיטא? מהו דתימא לרבי ספוקי מספקא ליה אי על כתב ידם הם מעידים, או על מנה שבשטר הם מעידים.

The Talmud asks: That is obvious. No analysis is necessary to arrive at this explanation of the dispute. The Talmud answers: The analysis is necessary lest you say that according to R' Yehuda HaNasi there is uncertainty whether they are testifying about their handwriting or whether the witnesses are testifying about the sum of 100 dinars that is in the document, and due to the possibility that the purpose of the testimony is to authenticate their handwriting, he requires two witnesses for each signature.

ונפקא מינה, היכא דמית חד מינייהו, לבעי שנים מן השוק להעיד עליו.

And the practical difference between whether the opinion of R' Yehuda HaNasi is based on certainty or uncertainty is in a case where one of the witnesses who signed the document died. If his opinion is based on certainty that they are testifying about the signatures, one other witness testifying to the authenticity of both signatures would suffice, as both that other witness and the surviving signatory would testify to authenticate each signature. However, if his opinion is based on uncertainty, let them require two other witnesses from the street to testify about the signature of the deceased witness.