It seems like its mostly meant to be symbolic of the duality of human nature and that we all have good and bad sides.
Tying in the description of CA and NV by Miles, perhaps the message could be that in our pursuit of happiness and fortune, we can't really have both unless we face our demons and reconcile our dual nature. This is seen in the character of Father Lynn who is really a thief, but becomes the priest at the end in order to give peace to Miles and is then able to escape with the money and Darlene. But at this point I'm mostly just making up my own interpretation.
Some examples of dualities that come to mind:
- The state line and symmetry of the hotel itself
- Playing roulette and making the hostages choose red or black
- Showing scenes multiple times from different points of view
- The priest wears black until the very end in Reno he is wearing white
- Billy Lee's speech about right vs wrong
- Darlene eventually takes off her wig (revealing her true self)
- And I'm sure there are more that I'm not remembering...
I think the following review from ScreenCrush says it best:
Everywhere you look in Bad Times at the El Royale you’ll see doubles. None of the characters are who they initially appear to be. They all hide something, often a second, secret identity. Each of their rooms at the El Royale Hotel in Tahoe comes furnished with an enormous mirror, the better to enhance that sense of multiple selves. And the hotel itself is split into two mirrored halves — it sits on the state line between Nevada and California with each state housing an identical half of the building. Lines of dialogue between characters are often spoken twice; scenes are repeated from different points of view.
Doubles, doubles, doubles everywhere — up to and including this movie’s runtime, which at two hours and 20 minutes feels about twice as long as it needs to be. Bad Times at the El Royale tells a fairly simple crime story in the most convoluted fashion imaginable, interrupting itself constantly with flashbacks and tangents and stories that do nothing but slow an already deliberate film down to a standstill while symbolism drips off every frame like a salad that’s been overdressed with blue cheese. It adds nothing to the movie beyond the basic message that most people in this world are not who they appear to be. If writer/director Drew Goddard was trying to say more than that, whatever it was got drowned out by the violence and the rain and the heavy-handed metaphors at the El Royale.
The only comments I can find from Drew Goddard about this comes from an interview with Whatculture:
Q: The film seems to explore the idea that we do all have our dark sides and most of us have goodness as well?
A: “I think that’s true and that is a part of this film. That is part of the reason why I began with archetypes for the main characters. There is the tradition of the lounge singer, the priest, the vacuum cleaner salesman, the concierge and the dark outlaw with a mysterious past. It was very important to me to start with the archetypes and explore them as actual people, rather than as storytelling devices. That was the inspiration for the film and I think through that the characters reveal themselves and the themes of the movie reveal themselves.”
Regarding the hotel itself --
Q: Can you discuss the style and look of the film?
A: “It was interesting getting to design my own hotel, and it was very much a collaboration with my longtime production designer Martin Whist. The El Royale is based on our asking, ‘If we could design our dream hotel, what would it be?’ It’s perfectly symmetrical, half in California, half in Nevada. Each side is inspired by the characteristics of its corresponding state. We tailored the rooms to the themes of the states, and I love both states. I live in California now and I go to Nevada often so it’s very much a love letter to those two states and the fun of having a hotel that straddles the border. California is about hope and opportunity in my mind, so it’s a much more vibrant and warmer side.
We have fun with the idea that there are different laws in terms of liquor licenses in Nevada, so you can only drink alcohol on the California side of the hotel. There’s also gambling on the Nevada side. We play with location as much as we can.”