Gemara

2b - Question: According to Rabah, why is a Shali'ach believed to say that the Get is Lishmah? We should require two witnesses, like all testimony!

Answer: One witness is believed about Isurim (what is forbidden).

Objection: We say so when no Isur was Muchzak (established), e.g. a piece of meat that is Safek Chelev (forbidden lard), Safek Shuman (permitted lard). One witness cannot permit a woman who was Vadai married (and forbidden to all other men)! Also, two witnesses are needed for Devar sheb'Ervah (Isurei Bi'ah)!

Answer: Most people know about Lishmah, in particular all the scribes, who write (almost all) Gitin. (Mid'Oraisa, no testimony is needed.) It is a mere stringency mid'Rabanan that the Shali'ach must testify about it.

3a - Question: According to Rava, why is a Shali'ach believed for Kiyum (to validate the signatures)? Kiyum of documents requires two witnesses!

Answer: Letter of the law, there is no need for Kiyum at all. Reish Lakish taught that a signed document is like testimony accepted in Beis Din. It is a mere stringency mid'Rabanan that the Shali'ach must testify about it.

66b: A man can tell two people to ask a scribe to write a Get, and to ask two others to sign it.

Yevamos 39b: Rav Yehudah enacted to write in a document of Chalitzah '... it became known to us that he is the Yavam...'

Rav Acha and Ravina argue about whether 'it became known to us' must be through witnesses, or if it can even be through a relative or a woman.

The Halachah is, this is a mere Giluy Milsa (a matter that is known not only to people who witnessed a certain event), so a relative or woman suffices.

Rishonim

Rif (Yevamos 13a): 'It became known to us' is a mere Giluy Milsa. Therefore, witnesses who heard that this is the Yavam and this is the Yevamah may rely on this for matters of Isur or money, unless there is reason to be suspicious.

Rambam (Hilchos Sanhedrin 24:1): The Torah requires two witnesses for a judge to rule according to them even if he does not know whether the matter is true.

Rambam (Hilchos Edus 3:4): The Torah accepts testimony only through the mouths of two witnesses. Mi'Divrei Sofrim we rule about monetary cases through testimony in a document, even if the witnesses are not around, lest people be loathe to lend.

Question: How can a man tell witnesses to tell other witnesses to sign a Get? Their signatures are like Ed mi'Pi Ed (testimony that one knows only through other witnesses, which is invalid)!

Answer (Tosfos Gitin 67a DH Imru): Ed mi'Pi Ed is when two people testify about a loan that others witnessed. The testimony was not entrusted to those who testify, rather, to those who saw it. Here, the testimony was entrusted only to those who will sign, not to the Sheluchim!

Question: We never find that one may tell Sheluchim to make other people witnesses about a loan or sale!

Answer #1 (Pnei Yehoshua Gitin 29a DH Acharei): Regarding money, we are concerned lest the Sheluchim are lying. We are not concerned lest a woman hire false Sheluchim, for one who remarries with a Pasul Get is fined greatly.

Answer #2 (Nesivos ha'Mishpat 28:7): Here is different; this is not final testimony. They will not give the Get until they hear from the husband, or they will return it to him. When he gives it to her, it is as if the signed witnesses testify to the giving, or (when we see her with a signed Get) we are (like) witnesses that he gave it to her. Tosfos teaches that the latter witnesses can believe the Sheluchim, for the only testimony that must be in Beis Din is Devar sheb'Ervah, or monetary testimony that completes the case. Here, they testify only that the Get can divorce. The identity of the Yavam, or the presence of signs of adulthood in a girl, is not Devar sheb'Ervah (Rif, Ramban Nidah 48b), even though later it will pertain to Devar sheb'Ervah or capital cases. Likewise, a man's authorization to sign a Get is not Devar sheb'Ervah, so the Sheluchim need not testify in Beis Din. Since there is Chezkas Isur (she is married), we need two witnesses, but this need not be in Beis Din. The Pnei Yehoshua says similarly.

Rebuttal (Meshovev Nesivos 39): The Pnei Yehoshua says that Sheluchim can appoint witnesses to sign a Get only because she is afraid to hire false witnesses. This is unlike the Nesivos!

Poskim

Shulchan Aruch (CM 28:12): We rule about monetary cases through testimony in a document, even if the witnesses are not around, lest people be loathe to lend.

Shach (14): This is from the Rambam, who says that this is an enactment. The Tur, Shulchan Aruch, SMA and others agreed. The Ramban (Hasagos on Sefer ha'Mitzvos) refuted this with clear proofs. E.g., Gitin 2b considers two witnesses signed on a document to be valid testimony mid'Oraisa.

Rema (CM 38:1): The Tur wrote many laws about Edim Zomemim (we learned that they were not present to see what they testified about). Even though nowadays they are not fined, they become disqualified from testifying.

Ketzos ha'Choshen (4): If witnesses testified that Reuven's wine was poured to idolatry and they were Huzmu, they must pay for it. Even though they testified about wine, not "Achiv", we find that Edim Zomemim about bestiality are killed and pay the value of the animal (which would have been killed),

Rebuttal (Nesivos ha'Mishpat 2 DH v'Ayen): "Ka'Asher Zomam" applies only to something that needs testimony in Beis Din, e.g. to stone an ox. However, even when Isurim require two witnesses, they need not testify in Beis Din.

Defense (Meshovev Nesivos): Even one witness, who need not testify in Beis Din, can retract and say that he lied until he testifies in Beis Din (Teshuvas ha'Ran 47). This shows that testimony in Beis Din applies. If the testimony pertains to the owner, if they are Huzmu, they pay. Kesuvos 26a shows that testimony in Beis Din is required to say that a Kohen is Pasul because his mother was divorced. The Tosfos Rid calls this Devar sheb'Ervah, for which the Nesivos agrees that Beis Din is required, but the Rambam holds that it is not Devar sheb'Ervah. Still, it must be in Beis Din, and Hazamah doesn't apply only because we cannot disqualify the witnesses' children like the witnesses plotted to do to the Kohen's children (Makos 2a). This shows that Hazamah applies to Isurim. When one witness is required for testimony, e.g. for a Korban or an oath, he must testify in Beis Din.

Shev shematsa (6:13): A Tosefta (Sanhedrin 6:4,5) says that witnesses who said that something is Tamei, Tahor, forbidden or permitted can retract and say that they were lying as long as their testimony was not investigated in Beis Din. This is difficult, for testimony about Isurim need not be in Beis Din! Rather, it means after the testimony was finished.