If I may hazard a potentially lighter note, when I first heard of the premise of China Miéville's book The City & The City, I had a hard time wrapping my head around it: two cities that occupy the same space but consciously ignore one another. Then I realized there was a perfect real-life analogy out there: black St. Louis and white St. Louis.

Sure, it's easier to segregate when you have physical separation, but not necessary. As long as you keep your interactions on the most basic level you need to, two societies can rumble along side-by-side without really knowing or understanding one another. It's more obvious in cities with a very mixed population, but I've never seen a fully integrated city, at least not in this country.