Eruvin, Chapter 7, Mishnah 2. The Mishnah continues with the topic discussed in the previous Mishnah: when two courtyards may join in an eruv and when this is not possible.
"Kosel shebein shtei chatzeiros gavoha asarah v'rochav arba" - a wall between two courtyards that is ten handbreadths high and four handbreadths wide:
The Gemara clarifies that the detail "and four wide" is not relevant to the first part of the Mishnah, and it is mentioned here only in passing, because we will need it later on. For the first part it is enough that the wall be ten handbreadths high, and then - "me'arvin shnayim v'ein me'arvin echad" - they make two eruvin and may not make one: each courtyard makes its own eruv separately, and they cannot join in a single eruv, since a wall ten handbreadths high separates them.
"Hayu b'rosho peiros" - if there was fruit on top of the wall:
What is the law when fruit was lying on the width of the wall on Shabbos? The Mishnah says: "Eilu olin mikan v'ochlin, v'eilu olin mikan v'ochlin" - these go up from this side and eat, and those go up from that side and eat. Even though they cannot join in one eruv, the residents of both courtyards may climb up to the top of the wall and eat the fruit lying there. However, "uvilvad shelo yoridu l'matah" - provided that they do not bring the fruit down with them when they descend.
The Gemara's clarification of the ruling "that they not bring it down":
The intention is not that it is forbidden to bring the fruit down into the courtyard, for we rule like Rabbi Shimon in the ninth chapter, who holds that roofs and courtyards are considered one domain with regard to objects that were lying in them from the onset of Shabbos. The top of the wall is considered a roof in this case, and the fruit was lying on it from the onset of Shabbos, and therefore it is permitted to move it into the courtyard. But it is forbidden to bring it into the house, and that is what the Mishnah means: one may not bring the fruit down and take it into the house.
This case is one of the reasons the Mishnah specified that the top of the wall is four handbreadths wide: since it is four handbreadths wide, it takes on the status of a domain in its own right. Had it been less than four handbreadths wide, its status would be that of a makom petur, which is not considered a domain in its own right, and then it would have been permitted to take the fruit from the top of the wall, bring it down, and even bring it into the houses of those courtyards.
A wall that was breached:
Let us return to the principle that a wall ten handbreadths high between two courtyards prevents them from joining in one eruv. What is the law when the wall was breached and an opening was created in it?
"Nifratz hakosel ad eser amos" - the wall was breached up to ten cubits: as long as the width of the breach does not exceed ten cubits, "me'arvin shnayim v'im ratzu me'arvin echad" - they make two eruvin, and if they wish they make one. They may continue to make two separate eruvin, and, like the law of any opening between two courtyards, if they wish they may make one joint eruv. The reason: "mipnei shehu k'pesach" - because it is like an opening. Even though no opening was made there originally but it was breached on its own, its status is that of an opening.
"Yoser mikan" - more than this: when the breach is wider than ten cubits, "me'arvin echad v'ein me'arvin shnayim" - they make one eruv and may not make two. Here they have no choice, since the wall is no longer considered a wall, and the two courtyards are considered one courtyard. There is no halachically significant partition between them that would allow them to separate and make two eruvin, and therefore they must join in one eruv.