Gemara

(Beraisa): The Torah expresses divorce using the verb Sholach (sending) to teach that a man can make a Shali'ach (agent). "V'Shilchah (read without a Mapik (dot inside the) Hei, i.e. she will send)" teaches that she can make a Shali'ach Kabalah (to receive her Get).

The repetition of "v'Shilchah" teaches that a Shali'ach can make a Shali'ach.

Question: This teaches about divorce. What is the source for Kidushin? We cannot learn from divorce, for divorce works against a woman's will!

Answer: "She will leave... and she will be (Mekudeshes)" equates divorce and Kidushin. Just like he can make a Shali'ach to divorce, also to be Mekadesh.

Gitin 66a (Mishnah - R. Meir): If a man asked two people to give a Get to his wife, or he asked three to write and give a Get to his wife, they must write it and give it themselves. If he asked three to give a Get to his wife, they may tell others to write (and give) it, for it is as if the husband made them a Beis Din. R. Chanina of Ono testified that R. Akiva also said so.

R. Yosi: We have a tradition, that even if he asks the great Sanhedrin, they cannot appoint someone else to write it. They themselves must write it.

Rishonim

Rambam (Hilchos Gerushin 7:4): If a Shali'ach to give a Get within Eretz Yisrael got sick, he can appoint another Shali'ach to give the Get.

Beis Yosef (EH 141 DH u'Mah sheha'Shali'ach): It seems that the Rambam holds that even her Shali'ach can make another Shali'ach, for he does not distinguish. Gedolei Acharonim say that Mili Lo Mimseran l'Shali'ach only regarding signing on a Get, which another cannot do. A Shali'ach can appoint another Shali'ach. I say that just like a woman can make a Shali'ach Kabalah for Kidushin can make another Shali'ach, like we expounded in Kidushin, a Shali'ach Kabalah for a Get can do so. We equate Kidushin and divorce.

Note: The Gemara expounds about divorce, and learns to Kidushin!

R. Baruch (cited in Mordechai Gitin Siman 420): Why does it say that Mili Lo Mimseran l'Shali'ach? In Kidushin we learned that a Shali'ach can make another Shali'ach, even though it is Mili! Some say that receiving a Get or Kidushin is a full act (it is not Mili). My Rebbi said that anything that the Shali'ach does not finish is called Mili, and cannot be transferred to another Shali'ach. The completion of a Get is not writing it, rather, giving it. Even if people were made Sheluchim for everything (writing and giving), the Shelichus to write is Mili. R. Meir and R. Yosi argue only about one who told a Beis Din. All agree that if one told two, who are not a Beis Din, they cannot make another Shali'ach.

Mordechai (Kidushin 505, cited in Beis Yosef EH Sof Siman 35 DH Kosav): The Kadosh me'Redush says that if one gave a ring to a Shali'ach to be Mekadesh, even if an Ones occurred, the Shali'ach cannot make another Shali'ach, for Mili Lo Mimseran l'Shali'ach. Even though a Shali'ach with a Get can make another Shali'ach, and here he has the ring, divorce is different. It is not MIli, for it works b'Al Korchah (against the woman's will). Kidushin needs her consent, so it is Mili.

Question (Bach DH Kosav): The Gemara equated Kidushin to divorce! R. Baruch does not distinguish them. There is no source to say that we learned only that a man can make a Shali'ach for Kidushin, but his Shali'ach cannot, for it is Mili. The Gemara did not say that it depends on b'Al Korchah!

Rashba (41a DH v'Shilach and DH Melamed): We apply the 'Vov' to 'Shilach' and 'Shilchah', to teach that his Shali'ach and hers can make another Shali'ach. This is Mili! Ha'Rav Bartzeloni says that she made a Shali'ach with a Kinyan, which is stronger. Presumably, she authorized him. Then we do not say Mili Lo Mimseran.

Tosfos Rid (41a DH sheha'Shali'ach): Her Shali'ach cannot make another Shali'ach, for it is Mili. She merely said words to him. It is unlike his Shali'ach, for he gave something (a Get). A Shali'ach to be Mekadesh cannot make another Shali'ach. Even if he gave an item (for Kidushin), it is Mili. It is unlike a Get. If a Get was lost, the Shali'ach cannot divorce. If the item for Kidushin was lost, the Shali'ach can Mekadesh her with his own money.

Poskim

Rema (EH 35:6): Some say that if Moshe was Levi's Shali'ach to be Mekadesh, Moshe can make another Shali'ach if he gives over the Kidushin (money or item) that Levi gave to him. If Levi gave nothing, rather said 'be Mekadesh her with whatever money you want', Moshe cannot make another Shali'ach. Some say that in any case he cannot make another Shali'ach.

Shulchan Aruch (EH 36:5): Some say that a Shali'ach to be Mekadesh cannot make another Shali'ach, for it is Mili.

Chelkas Mechokek (7): The Shulchan Aruch did not distinguish between a Shali'ach to give or receive Kidushin, and whether he received a ring. This is like the Kadosh me'Redush. In any case, he cannot make another Shali'ach.

Beis Shmuel (4): The Shulchan Aruch connotes that only a Shali'ach to receive Kidushin (like the previous Sa'if (4) discusses) cannot make another Shali'ach, but a Shali'ach to be Mekadesh can make another Shali'ach.

Shulchan Aruch (141:43): A Shali'ach Kabalah for a Get cannot make another Shali'ach, even if there was an Ones, and even if she authorized him to do so, for it is Mili, and Mili Lo Mimseran l'Shali'ach. Some permit if she authorized him.

Tur: A Gaon said that a Shali'ach Kabalah can make another Shali'ach only if he was explicitly authorized. He can make a Shali'ach to bring the Get, for this is from the husband's power.

Beis Yosef (DH She'elah): The Ba'al ha'Itur explains that a Shali'ach Kabalah can make another Shali'ach to bring the Get to the Shali'ach Kabalah. She cannot be divorced through the reception of the second Shali'ach.

Beis Shmuel (59 and Taz 26): A Shali'ach to bring a Get is not Mili, for he received a Get. A Shali'ach to bring a Get is not Mili, because the mission is finished when he gives over the Get to the Shali'ach according to her words.'

Pischei Teshuvah (45, citing Yeshu'os Yakov 19): A Shali'ach to bring may make another Shali'ach only after he received the Get. Beforehand it is Mili.

Toras Gitin: The Rambam holds that her Shali'ach to bring a Get is a Shali'ach Kabalah, with a stipulation that she is not divorced until she gets the Get. This is Mili, but after receiving the Get he can give it to another, since it is merely to fulfill the stipulation. The Beis Shmuel holds like those who disagree, and say that she cannot make a Shali'ach to bring her Get. Her husband must make him a Shali'ach, therefore it is not Mili.

Rema: In such a case he can make another Shali'ach in front of two, without Beis Din.

Beis Shmuel (60): This is even if there is no Ones.