Kesubos, chapter two, mishnah 4. Here we have a further discussion about the verification of documents, this time focusing on authenticating the signatures of the witnesses on the document.
The first case - each witness confirms both signatures:
"Zeh kesav yadi v'zeh kesav yado shel chaveri", "Zeh kesav yadi v'zeh kesav yado shel chaveri" - harei eilu ne'emanim - "This is my handwriting and this is the handwriting of my fellow," "This is my handwriting and this is the handwriting of my fellow" - they are believed. Each of the two witnesses confirms that the signature before him is his own, and goes on to confirm that the second signature is that of his fellow, the other witness. The result is that two witnesses testify about each and every signature, so the signatures are authenticated, there is no concern of forgery, and the debt can be collected on the basis of the document.
The second case - each witness confirms only his own signature:
What is the law when each of the witnesses says only "This is my handwriting," without confirming his fellow's signature? On this the Tannaim disagreed:
The opinion of Rabbi: "Tzerichin l'tzaref imahem acher" - they need to join another witness with them, meaning that an additional witness must testify about the signatures, so that there will be two witnesses for each signature.
The opinion of the Chachamim: "Ein tzerichin l'tzaref imahem acher, ela ne'eman adam lomar zeh kesav yadi" - they do not need to join another witness with them; rather, a person is believed when he says "This is my handwriting." There is no need for a second witness on that signature, and it is enough that the witness himself testifies about his own signature.
The root of the dispute:
According to Rabbi: a witness who confirms his signature is testifying about the signature alone. Since verifying a signature requires two witnesses, and he is only one witness, an additional witness must be joined with him.
According to the Chachamim: a witness who verifies his own signature, since he himself is the one who signed the document, is not merely confirming the signature but the content of the document. In a loan document, for example, he is confirming the loan itself. The result is that the two witnesses are testifying to the validity of the document not by force of the signature, but by force of their memory of the loan itself, and therefore no second witness is needed for the signature.
In summary: this mishnah deals with authenticating the signatures on a document. When each witness confirms his own signature and that of his fellow, they are believed, since two witnesses testify about each signature. When each witness confirms only his own signature, Rabbi and the Chachamim disagree: according to Rabbi an additional witness must be joined, because all we have here is testimony about the signature; according to the Chachamim there is no need for this, since a witness's testimony about his own signature is testimony about the entire content of the document.