It's intersting to me to see the replys about the fields cancelling each other on the same rotor. Perhaps thicker steel might help with that.
But the 1st thing to consider in my opinion about this....
given the option (if diameter is not a problem) - you'll get much more power from the same magnets and wire... to stick with a single machine, rather than stacking multiple smaller machines.
Double the number of magnets on 1 disk (make it larger diameter), and you'd have twice the number of pole changes per revoltuion, so each coil makes twice as much power. Double up the number of coils on one stator, (make it larger diameter) and you'll have twice the power. So I think doubling the number of magnets, and coils in one alternator will yield 4 X the power, where using the same resources, and basicly building two alternators on one shaft would only yield twice the power. (I think thats right...)
so unless the diameter of the alternator is an issue, I don't see much point in having multiple rotors and stators, best to build it all into one.