I took a run at writing this concept a while back and gave it up because I hit a technical snag. The intention was to alternate chapters between the two universes. The problem I very quickly discovered is that the fantasy side, Vaul, because it required extensive world-building, also required a hell of a lot more exposition than our usual universe. Exposition slows things down. So I’d end up with half the narrative going 60 mph and the other half doing 20.
By jumping in and going on instinct, and being a bit lazy frankly, I stumbled into a solution. Bring someone across from earth-side to Vaul right up front. That way I can use that character to stage the expo dumps. It’s so obvious it earns a duuuuuh. I have two Swiss watches, one runs fast, one runs slow, by inserting a crossover character I, in effect, add an extra gear between the workings of the two watches.
Gosh, I’m clever. Eventually.