By efficiency you are thinking in terms of effective use of magnetic flux. You are not meaning alternator efficiency in terms of watts out /watts in.

Magnets should be operated in closed or nearly closed magnetic circuits that concentrate the flux where it is needed. I take it that you are considering the axial alternator here. Magnets need to be mounted on steel discs to complete the magnetic circuit at the back. Plastic is useless and you will need monster magnets to do the same thing. For a rotating machine you will have to have an air gap to make it rotate now the problem is to get lots of copper into that air gap and still have a small gap for the magnet to excite.

The earliest machines just had open coils but later it was found beneficial to fix the coils to the surface of a laminated iron cylinder with poles round the outside.  This was a big improvement as you had a closed magnetic circuit except for the rather large air gap with the coils in it. Pacinotti had the brilliant idea of cutting slots in the laminated rotor core and putting the coils in the slots. This meant that the air gap could be reduced to a tiny value to give mechanical clearance and the magnets could be used far more effectively. Early permanent magnets were virtually useless and were soon replaced with electromagnets which needed lots of power to produce strong fields across large air gaps. The slotted core design became standard for about 100 years.

It still gives the smallest and cheapest alternator even when used with modern permanent magnets but the iron core has certain disadvantages for wind power and with the introduction of modern neodymium based magnets there are some advantages to going back to the old concept of having the coils in the air gap without teeth. This removes all iron loss and makes any form of cogging impossible. Neither of these things are of much importance for normal generators designed for maximum efficiency at full load.

For wind you want maximum efficiency at minimum load ( low wind). Unless you do something very clever you can't get high efficiency at full load so losses there don't matter much.

So for the axial design you need closed magnetic circuits except for the air gap.

You could make it single rotor with a slotted steel core and it is equivalent to the normal radial. You can leave it with single magnet rotor and use a laminated iron disc to close the air gap as much as possible but eliminate the teeth and most of the iron loss and cogging or you can bring 2 magnet rotors face to face with the same advantage of closed magnetic circuit except in the air gap and you can eliminate all iron loss. Messing about adding bits of iron or other ferromagnetic material within the stator of a dual rotor adds all sorts of problems with little gain and you would be better off going back to the laminated core single rotor with teeth rather than mess up an otherwise excellent idea.

Hope this helps.