Bava Batra 165B

מתניתין!

with the Mishnah remains: Why does the Mishnah state that which is obvious, that a document with a single witness’s signature is not valid?

הא קא משמע לן ⁠– דשנים במקושר, כעד אחד בפשוט; מה התם ⁠– פסולא דאורייתא, אף הכא נמי ⁠– פסולא דאורייתא.

The Talmud answers that the Mishnah teaches us this: That the case of two witnesses signed on a tied document is similar to the case of one witness signed on an ordinary document: Just as there, in the case of a single witness signed on an ordinary document, the disqualification is by Torah law, so too here, in the case of two witnesses signed on a tied document, the disqualification is by Torah law.

תדע, דשלחו מתם חברייא לרבי ירמיה:

Ameimar added: Know that the signature of one witness on a document and the oral testimony of one person about it can be combined into a single testimony, as the colleagues of the academy sent a question from there, i.e., Eretz Yisrael, to R' Yirmeya seeking clarification of a baraita. The baraita states that the first tanna holds that the testimony of two witnesses to an event can be combined into a single testimony only if both of them witnessed the event together, while R' Yehoshua ben Korḥa holds that the testimony can be combined even if they saw the event one after the other. Furthermore, the first tanna holds that their testimony is accepted only if they both testify together. By contrast, R' Natan holds that the testimony of one witness may be accepted today, and when the other witness comes the next day his testimony may be accepted, and the two testimonies may thereby be combined.