I was just sitting here pondering a VAWT. Here's an idea I came up with. Build two VAWT out of a set of 55 gallon barrels that would turn opposite rotation, stack them on top of each other. Couple them with a gear box with bevel cut gears. One gear from the top, one from the bottom with another beveled gear from the side that engages both top and bottom gear. This would keep them in time and drive the rotor/ generator from the side gear. If I'm correct this side gears RPM would be twice that of the top and bottom gear.
Another idea that would be simular would be to couple them in the same fashion to keep them in time and mount a stator between the two with a set of magnets on the bottom of the top VAWT and the other on the top of the bottom VAWT, then it would give the effect of turning twice the RPM. Think it would work?