1) "THE" PARTNERS
QUESTION: The Perek and Masechta begins with the word, "ha'Shutafin" -- "the partners." TOSFOS in Kidushin (2a) proposes that a Perek begins with the definite article "the" only when the subject of the Mishnah is mentioned in a verse in the Torah. The Mishnah uses the word "the" to point out that it is referring to the subject that is mentioned already in the verse.
According to Tosfos, why is the word "the" justified in the phrase "ha'Shutafin"? Where are the laws of "Shutafin" mentioned in the Torah such that the Mishnah is referring to that verse when it says "the partners"? (TORAS CHAIM)
(a) The TORAS CHAIM answers that the definite article "the" of "ha'Shutafin" does not refer to "the partners" mentioned anywhere in the Torah, but rather to "the partners" mentioned at the end of the previous Masechta. As Tosfos (DH ha'Shutafin) writes, Bava Basra is arranged after Bava Metzia because the end of Bava Metzia discusses the laws of "Shutafin" in a case in which one partner owns the ground floor of a house and one owns the second floor.
(b) The CHASAM SOFER answers that the division of jointly-owned property is mentioned explicitly in the verse. The Torah commands judges to rule justly between "a man and his brother and Gero" (Devarim 1:16). RASHI there cites the Midrash which translates "Gero" as a neighbor and which explains "brother" to mean that the brothers are coming to court to divide property that they inherited.
2) THE PARTNERS WHO WANTED TO MAKE A "MECHITZAH"
QUESTIONS: According to the opinion that the "Mechitzah" mentioned in the Mishnah refers to a wall (and not a mere division), the Gemara asks why the Mishnah does not say, "Bonin Oso b'Emtza." Why does it have to repeat the word "Bonin Es ha'Kosel" if it already mentioned that the partners build a wall ("Mechitzah")? The Gemara answers that if it would have said "Bonin Oso," one might have misunderstood the Mishnah to mean not that the partners agreed to build a wall but that they agreed to make a division ("Chalukah") and they build "it" ("Oso"), meaning a simple fence ("Mesipas"), to demarcate the border when they divide the Chatzer. (This is how Rashi (DH Havah Amina) explains the Gemara's answer.)
The Mishnah therefore uses the word "Kosel" to teach that the partners agreed to build an actual wall and not merely to make a division. If one partner does not agree to build a wall, the other cannot force him to build it because "Hezek Re'iyah" is not considered Hezek.
(a) Why is the Tana of the Mishnah concerned that one might misunderstand the Mishnah in such a manner? Even if one would understand the Mishnah to be saying that if the partners agree to split the Chatzer they must build a Mesipas, it still would be evident that Hezek Re'iyah is not considered Hezek since it suffices to build a simple Mesipas! (MAHARAM)
(b) How could one possibly misunderstand that the Mishnah refers to building a Mesipas and not a full-fledged wall? The Mishnah continues and says that if the local custom is to build with "Gevil" bricks or other types of brick, then the partners must build the wall in such a manner. This clearly refers to a proper wall and not a Mesipas (which is comprised merely of poles thrust into the ground, as Rashi (DH v'Ta'ama) writes)! (TOSFOS DH Havah Amina; Tosfos explains the Gemara differently from Rashi because of this and other questions.)
(a) The MAHARAM answers that it is true that the alternate reading would also teach that Hezek Re'iyah is not considered Hezek. However, it would not teach the second point of the Mishnah, which the Gemara emphasizes in the next Sugya. The Mishnah is teaching that if one of the partners prevails upon the other to agree to build a wall, the second partner cannot claim that he agreed only to help build the wall but not to have it take up any space in his part of the land. Rather, they must build the wall equally on both of their portions of land. This point can be derived from the Mishnah only if the Mishnah is discussing a partner who persuades the other partner to build a thick wall (which takes up space) and not just a Mesipas (which takes up virtually no space). That is why the Mishnah emphasizes that they must "build a wall (Bonin Es ha'Kosel) in the middle."
The Maharam gives a second answer and says that if the Mishnah would have said that when they agree to divide the land a Mesipas suffices, there would have been no indication from the Mishnah that Hezek Re'iyah is not considered Hezek. Perhaps the reason why a Mesipas suffices is that the reluctant partner may claim that he agreed to have the land divided (even though it is "Ein Bo Kedei Chalukah") but that he did not agree to share the expenses of building a wall. However, when the Chatzer is large enough for one partner to force the other to divide ("Yesh Bo Kedei Chalukah"), perhaps one partner may force the other to build a wall because of Hezek Re'iyah. The Mishnah therefore emphasizes that even when the Chatzer has "Kedei Chalukah," each partner must agree to build the wall (because Hezek Re'iyah is not considered Hezek).
Another approach might be that Rashi does not mean that if the Mishnah would have said "Oso" then the word "Mechitzah" would have been understood to mean a "division." As Tosfos asks, this opinion already rejected the possibility of translating "Mechitzah " as "division." Rather, "Mechitzah" still would mean a physical partition, but it would not refer to a solid partition like a wall but to a minimal partition like a Mesipas (see CHASAM SOFER). Consequently, there would have been no indication from the Mishnah that Hezek Re'iyah is not considered Hezek, since the reason it is not necessary to build a wall is because both of the partners agreed that a Mesipas would suffice.
(b) The RASHBA answers the second question by saying that according to Rashi, had the Mishnah not said "Bonin Es ha'Kosel," one would have understood the words of the Mishnah, "in a place where the custom is to build with Gevil...," to refer to a different case from the case discussed in the beginning of the Mishnah. In the beginning of the Mishnah, the Mishnah refers to a case in which the partners agree only to divide the land, but not to make a wall between them. In that case a Mesipas suffices. The second case of the Mishnah refers to where they agree to build a wall between them. In such a case, they must build the type of wall that is customary built in that place. (According to this answer, the second part of the Mishnah teaches that they build the wall in the middle even in a case where the partner's agreement is necessary in order to build the wall. This is a weakness in the first answer of the Maharam, who says that one would not have been able to learn this Halachah from the Mishnah had it not said "Kosel" in the Reisha.)