If a Kusi is a witness on a Shtar it is Pasul. (1)
If a Kusi is a witness on a Get or a Shtar Shichrur it is Kosher.
The Tana Kama says that Matzah baked by a Kusi is permitted to be eaten on Pesach and even may be used on the first night of Pesach. (2)
R. Eliezer says that we may not eat Matzah on Pesach that was baked by a Kusi.
R. Shimon Ben Gamliel says that any Mitzvah that the Kusim are Muchzak to keep they are more meticulous in the performance of the Mitzvah than a Yisrael. (3)
According to R. Eliezer one Kusi that signed on a Get is Kosher only if a Yisrael signed on the bottom. (4)
The witnesses on a Get must sign in front of each other. (5)
Any Shtar that is written by a Nochri court is valid even if signed by Nochrim with the exception of a Get and a Shtar Shichrur. (6)
R. Shimon says that even a Get and a Shtar Shichrur are Kosher when written and signed in a Nochri court.
R. Eliezer holds that it is not necessary for witnesses to sign a Get but if they do sign it must be done properly and otherwise it is Mezuyaf mi'Tocho.
A BIT MORE
1. Because we suspect that they will sign falsely.
2. In order to fulfill the obligation of eating Matzah on the first night of Pesach one must eat Matzah that was guarded l'Shem Pesach. According to the Tana Kama we trust a Kusi that he guarded the Matzah l'Shem Pesach and therefore it may be eaten for the Mitzvah on the first night of Pesach.
3. According to R. Shimon Ben Gamliel even a Mitzvah d'Rabanan that a Kusi is Muchzak to keep he can be trusted that he will do it properly.
4. Even though R. Eliezer holds that Kusim cannot be trusted and we suspect that they will sign falsely, however if a Yisrael signed after him it is proof that this Kusi is a Chaver (Talmid Chacham) because if not the Yisrael would not have allowed him to sign before him.
5. This is a Gezeirah because of a case where the husband instructed 10 people that all of them should write the Get and in that case all of them must sign and otherwise the Get is Pasul.
6. According to Shmuel even a Shtar Matanah that was written and signed in a Nochri court is Kosher, even though the Kinyan is made by means of the Shtar because Dina d'Malchusa Dina and the Shtar is Kosher.
KUSIM AS WITNESSES
Rashi says that Kusim are suspected to sign falsely on Shetaros and therefore they are Pasul to be witnesses. The Pnei Yehoshua says that this reason is only true according to R. Eliezer who holds that Kusim cannot be trusted for any Mitzvah and a Kusi may only sign on a Get if a Yisrael signed after him. However according to R. Gamliel who allows two Kusim to sign on a Get the reason he is Pasul to sign on Shetaros is not because he is suspected to sign falsely because if that was the case he would be Pasul to sign on Gitin as well. Rather the reason he is Pasul for Shetaros according to Raban Gamliel is because he does not know the Dinim of Shetaros.
SIGNATURES ON A GET
Even though it is only required mid'Rabanan that witnesses must sign on a Get the Rabanan decreed that the witnesses must spell out their names when they sign. They also decreed that the witnesses must sign in front of each other and if they do not sign in front of each other the Get is Pasul. (Rambam Hilchos Gerushin 1:24)