You don't give any details of magnets, but I get the impression that your stator is a bit slow for 12v and a bit fast for 24v. You may have a stall issue but I am more inclined to think that you have very little wind.
I am inclined to think that going to dual rotor is not going to get you any increase in power and it will certainly give you stall issues at 12v.
You may get some advantage using dual rotor on the same stator at 24v, there again I have no idea if you are near stall.
At the present moment before you do anything else I suggest you increase the distance from the magnet rotor to the stator and try again at 12v. If it is lack of wind I doubt you will see any difference. If you are running near stall you may see some improvement.
I really think your way forward is to go dual rotor with a larger prop but if you are insistent on using pvc you are on the verge of more trouble.
Flux
