What if Gotham and Metropolis are actually the same place? Like they both exist in the same space at the same time, each completely parallel and unaware of the other side of its own nature, the light and dark of one city.
Gotham is all grit and darkness and Metropolis all shining sunlight, but both are incomplete, each missing the part they hide from themselves, the part that is the "other" city.
This is why Batman and Superman are both allies and rivals, respectful of each other, but wary. Really, they are each other. Pessimism and optimism; skepticism and belief; preparation and reaction; wariness and openness. The two sides of one complete being. Batman is Superman and Superman is Batman.
I know, right? Mind blown. Okay. I gotta go. Stopping by Atomic Empire today. See ya, man.