Baba Basra 165a - One Witness Signing and One Watching

The gemara has a discussion about the ability of two witnesses to combine and create a viable eidus to validate a contract when one witness is signed in a contract and the other is a live witness. Tosafos is not sure what exactly the live witness is testifying about. Tosafos suggest that the case is where the live witness does not see the loan, but sees the borrower delivering the contract to the lender. Tosafos explains - שהרי יש במסירת שטר זה שני עדי מסירה, שהעדם החתום הוא כעד מסירה דמוכיח שמסרו לו לוה למלוה שהוא לא היה חותם אלא בצווי של לוה, והעד שבעל פה מעיד שמסרו לוה למלוה. Tosafos holds that the witness who is signed in the document can actually work as an eid mesirah. The witness signs at the request of the borrower who is admitting to him that he will be mi'shubad to the lender by delivering this document. Therefore, it is as if we have two witnesses watching the delivery of the document from borrower to lender.

This concept that a witness signed in a document can function as an eid mesira, is the approach of the Ran in gittin (48 of the Rif). There is a big discussion how witnesses that sign in a document can function according to R. Eliezer who holds עדי מסירה כרתי, that the witnesses who watch the delivery make the gett or document go into effect. How can the signed witnesses, without eidei mesirah accomplish this? Ran explains - דר"א ס"ל דעדי מסירה בלחוד כרתי ועדי חתימה לא כרתי וכו' ומיהו מאי דמודה ר"א דעדי חתימה מהני, היינו משום דס"ל שהמסירה כורתת כל שיש בשעתה עדים, בין שהם מעידים על המסירה עצמה או על גוף הדבר וכו' דהוו להו עדי חתימה כעדי מסירה וכו' ובידוע שהבעל מסרה לה ונמצאו כאילו הן עצמן מעידים על המסירה. The Ran explains that we regard a witness signed in the document as if he is literally watching the delivery of the document, because the fact the the document is in the hands of the woman or the lender indicates that it was delivered by the husband or borrower.

It is interesting that Tosafos 3a in gittin has a different approach as to why and how eidei mesirah work according to R. Elazar. Tosafos holds that the eidei chasima imply that it was likely given in the presence of eidim, rather than serve themselves as eidei mesira. It seems that Tosafos in Baba Basra holds like the Ran, not like Tosafos in Gittin.