Gemara

(Rav Yehudah): If one witness saw Reuven eat wheat, and one saw him eat barley, this is a Chazakah.

Question (Rav Nachman): If so, we should also say that if one witness saw Reuven eat in the first, third and fifth years (of a six year period), and another witness saw him eat the other three years, this is a Chazakah!

Rejection (Rav Yehudah): There is different, for they do not testify about the same years. I discuss witnesses who testify about the same years.

Question: They argue about what he ate!

Answer: Since he makes a Chazakah through eating either, witnesses are not careful to see exactly what he eats.

Rishonim

The Rif brings the Gemara verbatim.

Rambam (Hilchos To'en 15:2): If one of the witnesses of Chazakah testified that Reuven ate wheat, and the other testified that he ate barley, their testimony stands, for a witness is not so particular about this. If one witness saw Reuven eat in the first, third and fifth years (of a six year period), and another witness saw him eat the other three years, the testimonies do not join, for they do not testify about the same years. He returns the land and the Peros.

Magid Mishneh: We discuss a place where people leave the land fallow. If not, even if the witnesses did not contradict the other, it would not be a Chazakah!

SMA (CM 145:2): Perhaps the Rambam discusses when they do not contradict each other, in a place where people do not leave the land fallow! The Tur (140:12) says that such testimonies join, but the Rambam could argue, for in such a place each witness must testify about consecutive years! We can answer that if so, the Rambam would not have concluded 'for they do not testify about the same years', rather, 'because they do not testify about consecutive years.' Had they both testified about years one, three and five, they would join.

R. Akiva Eiger (CM 141:4): We may infer that the Rambam holds like the Tur (and one opinion in the Shulchan Aruch) that a place where people leave the land fallow, eating consecutive years is not a Chazakah. This is why the witnesses do not join, even though they do not contradict each other!

Rosh (3:68): Witnesses are not careful to notice whether Reuven ate wheat or barley, but they would know whether he ate wheat or legumes.

Hagahos Ashri: Even though people distinguish between a white Manah (100 Zuz) and a black Manah (Sanhedrin 31a), that is because it is the essence of the testimony, so they must be exact. We are not concerned about Chazakah, for whether he ate wheat or barley, it is a Chazakah.

Poskim

Shulchan Aruch (CM 145:1): If one witnesses testified that Reuven ate wheat for years of Chazakah, and the second testified that he ate barley, their testimony stands, for a witness is not so particular about this.

Shulchan Aruch (2): If one witness saw Reuven eat in the first, third and fifth years, and another witness saw him eat the other three years, do not join, for they do not testify about the same years. He returns the land and Peros.

Tur: If one witness (Moshe) says that he ate only years one, two and three, and the other (David) says that he ate only years four, five and six, he returns the land but not Peros. If they do not contradict each other, this is a Chazakah.

Prishah (DH Im): Even if David contradicts Moshe, but Moshe does not contradict David, it is not a Chazakah! Do not say that the Tur discusses when both contradict each other, for if not Reuven must pay Peros. In any case he need not pay Peros, for if we believe a witness that he ate three years, he has a Chazakah and the land was his! Rather, the Tur means that David or Moshe says that he ate only three years. Alternatively, the Tur says here that both contradict each other for parallel structure with the next case he brings (in which each witness testifies about alternate years). There, if David contradicted Moshe, but Moshe did not contradict David, David would obligate Reuven to pay for Peros. (Reuven cannot swear to contradict David, for he agrees that he ate!)

R. Akiva Eiger: I say that when each testifies about consecutive years, if David (the latter witness) contradicts Moshe, but Moshe does not contradict David, we believe David that Reuven has a Chazakah, Migo that David could have testified about the three years he saw Reuven eat, without mentioning that Moshe lied! The Ran (Kesuvos 10a) says that a witness is believed through a Migo, just like a litigant. The Pnei Yehoshua (Kesuvos 19b) says that the Ran discusses only a Peh she'Asar (had the witness been silent, he would have been believed), but this applies here!

Tur: If one witness says that he ate years one, three and five, and the other says that he ate years two, four and six, and they do not contradict each other, in a place where people leave the land fallow, some say that this is a Chazakah. I say that it is not a Chazakah, for if we believe both of them, he ate consecutive years, unlike the way people farm here. Also in a place where people do not leave the land fallow it is not a Chazakah, for neither testifies about consecutive years.

Beis Yosef (DH Hevi): The Rashbam says that Rav Nachman thought that Rav Yehudah meant that (we assume that) the witnesses testified about different years, for if they testified about the same years, they contradict each other. Rav Yehudah answered that we assume that they discuss the same years, but there is no contradiction. Tosfos rejected this (for if so, why does Rav Nachman say 'if so...?' Rav Yehudah teaches precisely this!) Tosfos says that Rav Nachman knew that Rav Yehudah meant that the witnesses testified about the same years, and asked that if this is a good Chazakah even though they contradict each other, we should likewise allow a Chazakah when they contradict each other about which years they ate. The Tur holds like Tosfos.

Drishah (2): Farmers told me that it is normal to plant wheat and barley in alternate years, and then one need never leave the land fallow. Tosfos need not argue with the Rashbam l'Halachah. They argue about even if the witnesses did not contradict each other. It is not a Chazakah if they do not testify about consecutive years. Rav Nachman did not want to ask directly, for this is not so obvious. Rather, Rav Nachman asked in any case when the witnesses explicitly contradict each other about the years. The Tur did not write a difference between Tosfos and the Rashbam, for he holds that they agree l'Halachah. In a place where people (alternate crops and) never leave the land fallow, all agree that using the field in alternative years is not a Chazakah. Both of them hold that the Gemara did not discuss a place where people leave the land fallow in alternative years. The Tur brings an opinion that in such a place, it is a Chazakah even according to the Rashbam. The Tur holds that even in such a place it is not a Chazakah. The Beis Yosef says that the Tur holds like Tosfos. It seems that he holds that they argue l'Halachah, and Tosfos holds that it is a Chazakah when they do not contradict each other. However, the Tur says that even then there is no Chazakah! Perhaps we can say like I explained. The Tur holds like Tosfos, that Rav Nachman asked from a case in which the witnesses contradict each other. 'Some say' (in the Tur) hold that Rav Nachman holds that when they do not contradict the other, it is a Chazakah in a place where people leave the land fallow. The Tur holds that Rav Nachman holds that even there it is not a Chazakah. Rav Nachman asked from a case that is clearcut. 'Some say' cannot hold like the Rashbam, for he holds that even without a contradiction, it is no Chazakah! However, I disagree, for 'some say' discuss where people leave the land fallow, and the Gemara (and Rashbam) discuss a place where they do not.

SMA (3): The Gemara and Tur did not mention Peros. The Rambam and Shulchan Aruch mention Peros. They must discuss a place where people leave the land fallow, and the witnesses did not contradict the other. Since he ate abnormally, the owner saw no need to protest. The witnesses do not really contradict each other. If they did, we could not force Reuven to pay for Peros! However, perhaps Peros refers to what is now attached.