Looks pretty good. You probably don't need to have written on the map that its 'upper' or 'lower' or anything like that, or that the hole in the first one leads down (that would be something for a POI).
As for splitting them... basically you have to find a height (z cooridate, the middle /loc number) when it would be safe to switch from one map to the next. It should be basically slightly below the lowest point of one level but above the highest point of the next level (i.e. find those two and split the difference).
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