Ketubot, Chapter Two, Mishnah Four
The previous mishnah discussed witnesses testifying about their signatures. This mishnah continues to discuss the subject.
[If] one witness says, This is my handwriting and that is the handwriting of my fellow, and the other [witness] says, This is my handwriting and that is the handwriting of my fellow, they are believed.
[If] one says, This is my handwriting and the other says, This is my handwriting they must join to themselves another [person], the words of Rabbi [Judah Hanasi].
But the Sages say: they need not join to themselves another [person], rather a person is believed to say, this is my handwriting.
Section one: Generally, two witnesses are required to create valid testimony in Jewish law. In order to validate a signature two witnesses are needed about each signature on the document. If each witness affirms his own signature and the other persons signature, then both signatures on the document have been affirmed by two people, and the document has been validated.
However, if each person cannot affirm the other signature on the document, they must find another person to affirm the signature. Note that one person can affirm both signatures, so long as he recognizes them. All of this is Rabbi Judah Hanasis opinion. He holds that the witnesses are actually testifying about their signatures and therefore we need two witnesses on each signature.
The Sages hold that a person is believed when he affirms his signature. Therefore, neither needs to bring someone else to join his affirmation. The Sages reason that the witnesses are actually testifying as to the contents of the document. Hence the two are in and of themselves sufficient.