power is related to the cube of the swept area, so it should be easy to figure out just where a single big rotor is better than a pair of smaller ones. a pair of 3 blade 10 footers is 157 feet swept. that makes 30 feet of thinner blades. a single turbine would have to be 14 feet across to have the same swept area. thats only 21 feet of thicker blades. points to the big mill.

however, i think that you will still need a tail behind this thing, and that is going to fight your attempt to yaw some. having the two props unequal seems like a bad idea, you have to make twice as many custom parts, and potential for more bobbing in the wind as one of them flexes. you also need two gens, or right angle drives, or a belt setup. twice as many rectifiers, etc. points to the big mill for simplicity.

so it comes down to only one reason i would ever do this- if i already had a mill, but i only had room for one tower, and i did not want to scrap the old mill.

Basically make it look like a 2 engine aircraft, or get fancy and a 4 engine bomber :)
 If it's useless junk for decoration only you can put up almost anything anywhere, but if it serves a usefull purpose people panic and you get hit with zoning, codes, restrictions, etc....

So, making a 2 gennie aircraft looking mill on one tower and hiding the wires inside the tower, it looks like a useless decoration or wind vane and probably no-one would complain about that in most areas! Plus it would look cool if nicely done.

In thinking about this alot, I have decided myself to keep it kinda small. Lots of stress on those pipes running out the sides, supporting the weight of the mill and the force of the wind trying to push them backwards. The more support you add the more you block the wind flow, plus more weight of course on the tower.

My thought is a small wing out each side, round body in the center over the tower, rear rudder as the furling. Perhaps to support the wings extend the tower a couple feet above the body, then run a thin guy wire to the outer wingtips.

Make sure it can't really fly in hard winds though!!
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nothing to lose