Eruvin 55B

נותנין קרפף לעיר, דברי רבי מאיר. וחכמים אומרים: לא אמרו קרפף אלא בין שתי עיירות.

One allocates a karpef to every city, i.e., an area of slightly more than 70 cubits is added to the boundary of a city and the Shabbat limit is measured from there; this is the statement of R' Meir. And the rabbis say: They spoke of the measure of a karpef only with regard to the space between two adjacent cities, i.e., if adjacent cities are separated by a shorter distance than that, they are considered one city.

ואיתמר, רב הונא אמר: קרפף לזו וקרפף לזו. וחייא בר רב אמר: אין נותנין אלא קרפף אחד לשניהם!

And it was stated that the amora’im disputed this issue. Rav Huna said: A karpef is added to this city and another karpef is added to that city, so that as long as the cities are not separated by a distance of slightly more than 141 cubits, they are considered one entity. And Ḥiyya bar Rav said: One allocates only one karpef to the two of them. Accordingly, Rav Huna has already stated that the measure of a karpef is added to both cities in determining whether they are close enough to be considered a single entity.

צריכא, דאי אשמעינן הכא ⁠— משום דהוה ליה צד היתר מעיקרא. אבל התם, אימא לא.

The Talmud answers: It is necessary for Rav Huna to state this halakha in both instances, as, had he taught it to us only here, in the case of the breached wall, one might have said that a karpef is allocated to each city only in that case because it had an aspect of permissibility from the outset, namely, the two sections originally formed one city. But there, with regard to the two cities, say that this is not the case and the two cities are only considered as one if they are separated by less than the measure of a single karpef.