It is a Machlokes Amora'im whether a Shali'ach must give the Get to the wife in front of two or three people.
Two witnesses are requires to authenticate a Shtar.
One witness is believed to testify about Isurim if it is not Ischazeik Isura.
Two witnesses are required to testify about a Devar Ervah.
Reish Lakish says that when witnesses are signed on a Shtar it is regarded as if their testimony was interrogated by Beis Din.
The requirement to authenticate a Shtar is mid'Rabanan and in a situation of a potential Agunah the Rabanan waive the requirement. (1)
Rabah says that the Shali'ach does not say b'Fanai Nichtav Lishmah because the Shali'ach is liable to omit one of the three words. (2)
A woman who is a Shali'ach is believed to say B'Fanai Nichtav ub'Fanai Nichtam even if she is bringing the Get for herself.
Rabah Rava holds that the Shali'ach may say that I recognize the signatures in lieu of saying B'Fanai Nichtav ub'Fanai Nichtam, but Rabah argues. (3)
According to R. Meir although a Get should not be written l'Chatchilah on something that is attached to the ground, b'Di'eved it is Kosher if it was detached before the witnesses signed on it.
The Tana Kama says that a Get that is written by the husband but lacks witnesses, or if it lacks a date, or if it has only one witness it is Paul but if she uses the Get a child that is born to her is Kosher.
R. Eliezer says that a Get without witnesses is Kosher l'Chatchilah as long as it was given in front of witnesses. (4)
A BIT MORE
1. Therefore it is sufficient for the witness to testify B'Fanai Nichtav ub'Fanai Nichtam and no further authentication is required.
2. Therefore he says only B'Fanai Nichtav and since it is only two words he is not liable to omit one of them.
3. Rava holds that the reason for saying B'Fanai Nichtav ub'Fanai Nichtam is to authenticate the Shtar. Therefore, if they say that they recognize the signatures it is sufficient. However Rabah holds that the purpose of B'Fanai Nichtav ub'Fanai Nichtam is to testify that it was written l'Chatchilah Lishmah.
4. Because witnesses are only required on a Get for Tikun ha'Olam because of the possibility that the witnesses who saw the Mesiras ha'Get will die.