What's the benefit or tradeoff of two magnet discs, one winding disc, vs. the other way?
To answer my own question, it involves the iron or other pole-pieces that duct magnetic field around and back through.
Two magnet discs can be made by bonding magnets to sheets of steel. No moving magnetic field (relative to steel), no eddy currents.
If two winding discs with magnets in between, the magnetic field through coils alternates, so steel discs holding coils would probably get eddy currents and dissipate power.
Brandnewb's coil holder appears to have something troweled in back. Just structural? Or a ferrite mixture? The electrically non-conductive ferrite mixture would serve to duct magnetic field that passed through a coil around and back through another.
But I think metal performs better than powder at low frequency? So the two magnet discs scheme might have a lower "resistance" for the magnetic circuit.
Another idea regarding windings -
Magnetic circuit is through magnets themselves, jumps a gap through air, passes through metal or ferrite, back through air, to another magnet.
Air is the worst magnetic "conductor", so you would like the air gap to be as short a path as possible, and as wide a cross section as possible.
Windings, copper, take up significant thickness. So the opening through center of wire coils is reduced in cross section. If filled with air, that is higher "resistance". If filled with steel/ferrite, a lower "resistance". By using steel/ferrite to duct magnetic field through middle of coil, then fanning out to wide area before jumping the gap, air gap can have as large a cross sectional area as possible. Gap is also as short as possible, with rigid material able to hold its shape, no danger of it bending and contacting rotating magnets.
Here is an example of a (radial configuration) stator. Note pole pieces have a narrow gap to slip in wire while winding coils. The pole pieces are something of a "T" shape, wide area for gap to rotor, small area to allow more winding volume.
These coils are also tall, which is OK with pole pieces. Yours will be flat to minimize air gap, but I don't know that you can get enough turns to get the amount of power transfer you want.
For your flat coils disk you may want to encapsulate in epoxy, with a flat surface pressed down as a "mold" to define a rigid flat surface.