The game I am play at the moment has 15 Civs still alive on a huge Inland sea map and the cities are force to settle close together. apart from my first 2 they are all 5 hexes away from each other.
there is a spot that has room for 2 districts that will effect 10 cities IF Toronto and the Great Engineer stacks. If not that number drops to 7.
I made a post a day or so ago asking if they stack, did not want to waste my Great person if they did not but no one has replied yet

so I went ahead and used him anyway but I cant tell if it effects the out cities or not. Because I am trading Luxuries with the AI every other turn.