But how would splitting a single coil into two coils next to each other impact the timber?
I think that IF (and only if) the side by side coils are noise cancelling, it would affect mostly the level when the two center strings are bent and pulled each over the other coil... We've already discussed here about side by side coils with opposite winding directions + magnetic fields, for their annoying tendency to create a dead spot between G and D strings because of phase cancellation... It's not an issue with bridge PU's but bending the G string above the neck pickup will mute the sound. That's why Fralin Split Blades have "overlaping" coils (just like each pole of a Zexcoil systematically lays under two strings).
If two coils side by side have the same polariy, they should remain noisy, of course.
In both cases, I'd bet for a bit more inductance frow two mini SC's side by side than from one single big coil wound with the same wire on a bobbin of the same height and width...
i'm not sure that I would dig this recipe from Haven pickups anyway: using two of the four coils should either be noisy, either an open door for the dead spot issue. Three coils would be noisy too and might sound uneven from bass to treble strings, volume wise. Four coils would just form a regular rails humbucker... but maybe there's an advantage in this idea that I've not seen, of course (if the four mags and/or rails are of different alloys, for instance).
Final rambling for the record - Pickups with multiple coils are not exactly new: 40 years ago (way before Zexcoils), a French boutique winder named MC2 was selling a "PLO" single coil shaped pickup hosting six coils. In my collection, I've also a "Rainbow" pickup based on this principle and from the same era - wound in NY by Armstrong and meant to have belonged to Steve Howe, if memory serves me... but if it's the case, Howe got rid of it, exaclty like Hadley Hockensmith stopped to use 4 coils HB's n his LP, mounting instead a set of... regular GFS humbuckers.
[Last sentences to take with a grain of salt since they translate old memories from an old brain.

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FWIW. I wish you all a nice day.