I will leave Dan to answer your specific questions about his machine, but I can make some comments.
Using a second rotor will indeed act as a return path for the flux and will use the magnets more efficiently.
If Dan has found that a single rotor with no return path provides the optimum for this design, you will need to make changes if you add a second disc, so don't do it without thinking of the consequences.
I suspect with a second disc you would match an 8ft prop quite well if you got things right.
If you add a second disc to Dan's machine and leave the stator as it is, it will cut in at too low a speed and stall badly with the 7 ft prop. You would need to reduce turns to raise the cut in speed and that would leave you room for thicker wire. With the same cut in speed you would most likely still stall with the 7 ft prop and would need to add resistance in the line. The power into the battery would be similar but the stator temperature would be lower and you could have a larger safety margin or you could raise the furling speed.
There are infinite variations you can make as long as you know what you are doing.
Both DanB and Hugh Piggot provide complete working designs that will perform well if copied exactly and those without detailed knowledge are well advised to follow without introducing modifications.
The more experienced can often adapt the designs for their particular needs and sometimes make improvements to suit available materials, but I strongly suspect that many make changes that don't enhance the final result as they don't fully understand the implications of change.
Flux
