Question: Our Mishnah argues with Shmuel! If a wide Chatzer was breached to a narrow one, the wide is permitted, and the narrow is forbidden;

[Shmuel said (Bavli 94a) that if the wall between two Chatzeros fell,] one may carry up to where the wall was!

Remark: (In an episode (94a), the wall fell and Shmuel commanded to spread a garment where the wall used to be. Really, he permits even without this.) The episode was so (he spread it for privacy).

Answer (Talmidim, in front of R. Yosi, in the name of R. Achiyah): Shmuel discusses this Shabbos [the wall was intact when Shabbos began]. Our Mishnah discusses the coming Shabbos.

Question: What is the case?

If they made one Eruv, both this Shabbos or the coming Shabbos should be permitted;

If they were not Me'arev at all, both this Shabbos and the coming Shabbos should be forbidden!

Answer (R. Bun bar Kahana, in front of R. La): The case is, this [Chatzer] was Me'arev by itself, and this was Me'arev by itself;

Even so, the wide was permitted, for it was not totally breached. The narrow was forbidden, for it was totally breached.