two layers are fine PROVIDED that they are SECURELY attached to the joining ribs with, perhaps, interlocking wood.

but if they are not interlocking, then ya had betta reinforce them to the ribs with glass cloth and epoxy or else, even with 4 layers, they'll rip out. include the wing's leading edge and the ribs all the way back to the spar in the structural strungthening because that is where the tortional forces end up transfering to upon landings.