For magnets 1/2" thick the flux density falls off from Br with zero gap to about 1/2 Br with about 1/2" gap.
The fall is a hyperbolic curve with a rapid drop with small increases in gap and a slower fall off as the gap tends to infinite.
With very small gaps you have high flux density but little winding space and the mechanical clearance wastes a lot of the useful space.
With very large gaps the flux fringes between magnets and doesn't completely cross the air gap except at the centre of the magnets. You have lots of winding space but loose some of the flux.
Somewhere there must be the best operating point which gives the best result. I don't think this is at all critical and air gaps from 1/2" to 1" work well enough.Probably an air gap of about 3/4" with a stator something like 5/8" will bring your flux density at about half Br or 600+ mT for most magnets.
There is no real point in two stators unless you are building something very special, a single winding is more effective in using materials but you can vary the stator thickness from just under 1/2" to over 3/4" without a lot of overall effect in performance as long as you get the turns and wire size right.
Basically small gaps, few turns but only enough space for thin wire. Large gaps, more turns and more space to use thicker wire. The one that comes out with least resistance for a given voltage will be the winner but it will not be at either extreme.
Flux