Tractate Ketubot - Chapter 2 - Mishnah 4 כתובות פרק ב׳ משנה ד׳

[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.