Please make the 2nd big city part of the endless dungeon. Like a big drow or dwarven city far below the surface, at the end of the dungeon. Whatever its inhabitants (maybe a mix of races, having resettled the ancient city ruins), it should be deep below the surface, and only reachable through the endless dungeon. This would give the endless dungeon a real purpose. Thanks for reading, Obsidian.
Edit: Clearing the endless path is not important here, the 2nd big city can have its entrance anywhere in the dungeon, e.g. as a supply and quest base.
The main suggestion is that the 2nd big city is subterranean.
A city below adds more variety than two big cities above ground.