Thanks for all the interesting replies! Given the response, I decided to make a whole community around this, hope you'll consider joining!
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I'm not familiar with those, so this might be a bad suggestion, but the short description makes me think this may still fit, have you read The City & the City by China Miéville?
It's set in two overlapping cities, whose inhabitants diligently disregard the other city's until they formally cross the borders, and it's a crime to do otherwise. It's a pretty compelling read imo!
I haven’t but it sounds like I should. Thanks for the rec!
Kraken, also by Mièville, is also somewhat of a match; as well as Neverwhere by Neil Gaiman.