Is that something similar to the days before the DFW-Fort Worth airport was built when Dallas listeners only tuned in Dallas stations and Ft. Worth listeners tuned in only the Ft. Worth stations even though both groups could hear the other city's stations clearly (mostly) in their own back yard.
Keep-in-mind that Dallas and Ft. Worth started as separate markets because most listening was to AM, and only three AM signals covered both 24/7. While some stations have upgraded or otherwise changed their coverage, that's still true today; only three AM's in DFW cover the entire market around-the-clock. Pensacola and Mobile may be separate markets for the same reason. No AM's cover both with what would be considered a citygrade signal at any time of the day. No operator could make a case for the two being the same market until, at least, the early 80's.
While I could see something of a necessity for that in the past (because of local news and weather carriage), it sounds odd today when most commercial radio stations eschew local news (their marketing surveys say listeners don't want to hear it) and the weather forecasts are usually prerecorded ahead of time ffor multiple stations in the group.
It probably goes back to two factors. The first is that the two really are distinct advertising markets. As I mentioned above, you couldn't historically market to both because you couldn't cover both until FM became dominant in the area. Even today, almost half of Mobile commercial FM's don't put usable signals into all of Pensacola. Also, unlike Dallas and Ft. Worth, you can tell you've left Mobile when going to Pensacola (and vice-versa). The two are about twice-as-far apart edge-to-edge, and their individual metropolitan areas don't have much growth toward each other. In DFW, you have a vast set of businesses, including many, if not most, car dealerships, in the so-called "Mid-Cities." Most of those businesses simply wouldn't bother with radio if they had to make separate buys for both cities. Mobile and Pensacola have very little commerce in-between them other than the tourist areas in Perdido Key and Orange Beach. The second is that people know when people are talking to them. Even if many of the stations carry much of the same syndicated programming, the audience can tell which stations are talking to them, and those are usually the ones they prefer. Before WDLT moved to 104.1, it was on 98.3, which didn't put a listenable signal into Pensacola. Cumulus also had 106.1 doing urban AC in Pensacola. Today, 106.1 is still doing urban AC, though 104.1 totally overlaps its coverage area. The two even have, or at least had, the same syndicated programs in AM and PM drive. 106.1 remains one of the most listened to stations in Pensacola (usually in the Top-3), and virtually no one listens to 104.1 there. The reason is because people in Pensacola can tell that 106.1 is talking to them, and those same people know 104.1 is not.