Gemara

(Rav Yehudah citing Rav): One can be Machazik in property of one who fled.

Objection (Shmuel): This is obvious, for a Macha'ah not in front of the Machazik is valid!

Question: Seemingly, Rav came to teach that a Macha'ah not in front of the Machazik is invalid. He already taught this. Why must he teach it again?!

Answer: Rav teaches that a Macha'ah is valid even if the two who heard it cannot tell the Machazik.

(Rav Anan citing Shmuel): A Macha'ah is valid only if the two who heard it can tell the Machazik.

Rav disagrees, for the two can tell their friends, those friends can tell their friends... (and the Machazik will hear).

39a (Rav Zvid): If Reuven told the listeners 'do not tell Ploni', they will not tell him, so the Macha'ah is invalid.

(Rav Papa): It is valid. The listeners can tell their friends, those friends can tell their friends... (and Ploni will hear).

39b (R. Chiya bar Aba citing R. Yochanan): Macha'ah must be in front of two.

(R. Avahu citing R. Yochanan): Macha'ah must be in front of three.

Suggestion: R. Avahu holds like Rabah bar Rav Huna, that if one repeats a matter said in front of three people, he does not transgress Lashon Hara. R. Chiya bar Aba argues, and holds that a matter said in front of even two (will become known), and if one repeats it he does not transgress Lashon Hara.

Rejection #1: No, all agree with Rabah bar Rav Huna. R. Avahu holds that a Macha'ah not in front of the Machazik is valid. Three are needed to spread the word everywhere. R. Chiya holds that a Macha'ah must be in front of the Machazik. Surely the Machazik will hear. it suffices for two to hear it.

Rejection #2: Both validate a Macha'ah not in front of the Machazik. R. Chiya holds that two suffice, for they can be witnesses (and they are likely to inform the Machazik). R. Avahu requires three, for we want everyone to find out.

(Rava citing Rav Nachman): Macha'ah must be made in front of two.

Rishonim

Rif and Rosh (3:28):Macha'ah is valid even if the two who heard it cannot tell the Machazik. The two can tell their friends, those friends can tell their friends... Macha'ah must be made in front of two.

Rosh: Even though Shmuel disqualifies Macha'ah in front of two who cannot tell the Machazik, and the Halachah follows him against Rav in monetary laws, here the latter Amora'im hold like Rav. It seems that Rav Papa holds like Rav. One could say that even Shmuel holds like Rav Papa, for the witnesses could go to the Machazik's city and tell others, who will tell the Machazik. Rav Papa could disqualify Macha'ah in front of old or sick people. However, presumably we do not distinguish. Since your friend has a friend, and they will tell others, we do not distinguish between the same city and another city. Others will make it known in the city of the Machazik.

Rambam (Hilchos To'en 11:6): If one commanded the witnesses of Macha'ah 'do not tell the Machazik', or on their own the witnesses said 'we will not tell him', the Macha'ah is valid. Even though they will not tell him, they will tell others, and word will reach the Machazik.

Rambam (7): To make a Macha'ah, one says in front of two...

Lechem Mishneh: The Rambam did not need to teach that the witnesses can be old or lame, for he already was Machshir a Macha'ah in which he told them not to tell the Machazik, for they will tell friends...

Rosh (30): If Macha'ah was made in front of the Machazik, even without witnesses, if the Machazik admits, the Macha'ah is valid and it negates the Chazakah. Two witnesses are needed only for testimony. Since he admits, there is no need for this. The Ramban disqualifies Macha'ah even in front of the Machazik and one witness, for the Machazik has a Migo. This is wrong. He is not believed due to a Migo, for he should have been careful with his document.

Ramban (39b DH Macha'ah): If Macha'ah was made in front of the Machazik and one other, perhaps the Machazik can say 'I was not careful with my document, for I reasoned that if I will not find it, I can deny the Macha'ah.' Also, I thought that you will not take me to Din, even though you said that you will, for you did not protest in front of witnesses.' I concluded that the Macha'ah is invalid. Migo he would be believed to deny the Macha'ah, he is believed to say 'I bought the land.' Even though you protested, I was not careful with my document.' This is unlike the ingot of R. Aba. (There, we say that if one must swear to contradict one witness, but he cannot, he must pay.) Here, one witness about land is worthless. If the Muchzak claimed Peros, there is no Migo (because the witness obligates a Shevu'ah). Even so, the Macha'ah is invalid.

Tumim (146:1): If we believe that the Machazik owns the land, why must he pay for Peros? We must say that there are no witnesses of Chazakah. The Muchzak's witness does not help for the land itself (in any case there is no Chazakah). The Macha'ah does not help for Peros if the Machazik denies it and swears to deny it, but if he admits to the Macha'ah, since he cannot swear to contradict the witness, he pays. However, if the Muchzak claims three years of Peros, he admits to the Machazik's Chazakah!

Nesivos ha'Mishpat (Bi'urim 1): The Tumim himself answered that there are no witnesses of Chazakah. The Muchzak has one witness of Macha'ah, and demands payment for three years of Peros. The Muchzak does not admit to a Chazakah, for he says that he protested! If the Machazik admits to the Macha'ah, he has no Migo to be exempt from the Peros, for he cannot swear to contradict the witness.

Chazon Ish (CM Likutim 21 l'Perek 3, DH uvi'Ketzos): Indeed, the laws are separate! He swears to keep the land, for he has a Migo, but he cannot swear (mid'Oraisa) about Peros, so he pays for them! However, after we ruled that he keeps the land, all future Peros are his.

Note: Perhaps the Tumim says that the Muchzak admitted to a Chazakah, for he admits that he did not protest in front of two witnesses. (The Ramban holds that the Machazik had no need to guard his document.)

Poskim

Shulchan Aruch (CM 146:1): Macha'ah must be in front of witnesses. Two witnesses suffice, even if they are old or sick and cannot go to inform the Machazik, for they will tell others, and others will tell others, until the Machazik will hear.

Shulchan Aruch (2): Macha'ah in front of one, even in front of the Machazik, is invalid, even if the Machazik admits to the Macha'ah. Since he could have denied the Macha'ah, he is believed to say 'I bought the land.' This is even if one witness contradicts him. Some disagree.

SMA (3): We do not say that the Machazik must swear (to contradict the witnesses), and since he cannot, he must pay. This is because one witness does not obligate a Shevu'ah mid'Oraisa about land. For an oath mid'Rabanan we do not say 'since he cannot swear, he must pay' (75:14).

SMA (4): The Rosh holds that we do not say that he is believed due to a Migo, for he should have been careful with his document. The Ramban holds that he can say 'since you did not protest in front of two, I thought that you were joking, so I was not careful with my document.'

R. Akiva Eiger: According to this, if the Machazik admits that Macha'ah was made in front of two, even though there are no witnesses here, all agree that he has no Migo, for he should have been careful. I do not understand the Rosh. Why is this Migo different than the Migo to say 'I bought it', for he could have said 'you never owned it'? (In CM 140:1, we say that if Reuven cannot prove that he previously owned the land, the Machazik keeps it even if he was not there for three years, even though he should have been careful with his document!)

Shach (2): The latter opinion validates Macha'ah in front of the Machazik even without any witnesses. R. Yerucham rules like this. v. Gra (8): The latter opinion holds that R. Chiya said 'two witnesses' only to argue with R. Avahu (who requires three). A proof is from the Gemara, which says that he requires two for testimony. Witnesses are needed only to refute liars (but not when the parties admit). The first opinion holds that we require testimony to remove the Migo.