On a smaller space station, if there's a kiosk that's within the range of multiple stores, like Conscientious Objects and a Casaba, could a single kiosk operate for both stores?

It is possible, although it should not be configured that way. Let's say that we had two Casaba stores next to each other. In the middle, exactly in the middle, they had an item kiosk, and both stores have a radius that expands and covers that item kiosk. That item kiosk would then show items for both stores. Of course, the configurations done by the designers would probably never allow that to happen.