I have a quick question before I go on... have you worked out the angles between the wings for each to run in its respective TSR?  If the wings are set at a given angle between the two then one will run well at a slower speed while the other is slightly stalled, creating drag.  As well, when the other is working well at a faster speed the inner one may be running out of its respective TSR, also creating drag.  I'm sure you've worked out the differences but it was just an observation from looking at the machine in a picture where angles cannot be easily seen.

    A little background on wing design and changes that create big differences in the way they perform.  I spent 10 years of my life building aircraft from scratch, very challenging and quite fun ( sometimes quite dangerous).  I had the opportunity to fly many different designs and each have their good and bad qualities.  The ones that really stand out in reguards to verticle wind turbines was one called the condor and another called Quicksilver.  Both of these were single surface wings with a large arch forming the air foil.  Although they were a very draggy design they performed quite amazingly.  At 22mph they would lift 600 lbs off the ground.   When I first took the quick up for some stall tests I found a very docile, easy to control airplane.  Also, it was very difficult to get it to stall even at extreem angles.  If there was power, even a small amount, it simply wouldn't stall.  I was up around 2500 ft and decided to see how it handled without any power ( good to practice if you fly with a 2 stroke engine ).  I held the stick all the way back in my lap and the nose went high... and stayed there with a slight buffet, I knew the wings were stalled because of the awful noise comming from the structure but it held its attitude and simply lost altitude.

   Anyway, without boring you further with fun flying.  I carried these wing designs along with me into the verticals since most the most power we try to extract is from winds in the range of 6-20 mph.  These wings are draggy but that, I believe, is why they work so well.  They produce an enormous amount of lift, they will not go fast, but they operate in a very large range of angles without stalling as well as really nice low wind performance.  In higher winds they will fail to produce good output.  In an airplane this isn't a good thing, the more power you give it all you do is burn fuel and it will maintain its slow flight.  In a wind turbine if it fails to produce in winds over 30 mph and sits there doing its normal output not gaining speed and eating up the power, this is a good thing ( built in controls ).  

  Thus the Lenz turbine was born.  Below shows a picture of the newest version of the wings which so far have been quite successful...

  Fat, draggy, lots of lift!

Not always:  The purpose is to extract work form the wind.  You do that by diverting it, of course.  But you can sometimes extract even more work from it by adjusting its speed.  That's what a wing does - forcing the air to take a longer path, flow faster, lower its pressure further, and thus generate more lift, on the topside of the single foil.

And that's why lift-type mills, both vertical (darrieus) and horizontal (the ones that look like propellors, rather than the drag-type that look more like fans or blowers) have thick blades.

I like your double-bladed design as shown above.  (I'll delve into your web site later.)  Between the picture and your description it sounds like you're doing two things that sailboaters have done for a while, but applying them to a mill to good effect:

First, you're using a doubled blade - like the jib and mainsail on a sloop, the simplest of the multi-sail designs.  The jib does two things:  It provides its own lift, of course.  But it also directs the wind against the backside of the mainsail.  This improves both the attachment and the airspeed on the lee-side airflow of the main, increasing the power of the main.  You get, more power, power going TOWARD the wind (though not straight into it) and with two sails you get power at a TIGHTER ANGLE toward the wind.  You also get a more forgiving adjustment on the sail angles, which, in a rigid blade configuration, amounts to getting good power for a wider arc of the rotation.  An important point, though, is that the extra blade gives you the ability to increase the speed of the air over the leaward side of the (trailing) blade using only thin blades, rather than requiring a thick structure.  So you get a lift-type turbine with thin blades.  (You might think of it as separating and reversing the top and bottom sides of a thick blade, though that's probably distracting from what's really going on.  B-)  )  It also means you can have a TSR greater than 1 and optimize your leading and trailing edges separately.

The experience gained with the device I have over the last 12 months it has been running is that (1) the doubled blades allow it to start up and run (no small feat!) instead of sitting there without flow attachment on the blades and refusing to do a thing in the wind and (2) making the doubled blades thinner allows it to run faster instead of just turning as slow as molasses. This has been a total revelation to me and I'll bet my excitement shows.

And diservedly.  The combo of the blades makes it act like a drag turbine at startup, then switch to a lift turbine as it speeds up.  Good work!  (Like a Darrieus with a Savonius starter rotor, but without the non-optimized Savonius in the way when you're up and running.)

Second:  Sails themselves are thin but (except for jibs) the mast provides a fat leading edge.  Your text sounds like you've done something similar.  (I'll dig into your web site to see if this is a correct reading.)

The other thing is that I am sure something can be learned in the case of the horizontals from this as well.

Perhaps.

But lift-type horizontals are already well-developed and pushing the wall on efficiency.  The Betz limit tells you the maximum power can be extracted from a given airflow, and they're within a couple percent of it, so there's not a lot of room for improvement.

In addition, the horizontals rotator also mentioned has been constructed and has run for over a month now. It has the same type of blades as the verticals rotator, thin aluminum sheets with a folded (rounded) leading edge and doubled blades with the offset.

Everyone agrees hereabouts who has seen it that it performs remarkably well (albeit just a free wheeling, nonpowered device), rotating at high rates of speed in light winds. The trailing edge pitch angle of all the blades is also a feature of it - zero degrees. Yes, it performs noticeably better than the verticals rotator for approximately the same blade swept area - one half of a square meter.

Doubled, sheet thin blades. A low thickness-to-chord ratio. This is a serious attempt to reduce parasitic drag and so far it has provided excellent results. It is important to understand that my approach has a name - the Newtonian Principle of Aerodynamics, a term coined by others. This is unlike the more inductive approaches typically seen in most flight-oriented aerodynamics work until just a few years ago. A look at what is on the www.integener.com website explains. Some nice photos are also to be viewed there of both rotators in action.

Thin blades. Thin blades. Thin blades. I have seen the difference in the actual reality of what has resulted with real hardware. Get rid of all flutter and flow separation off the leading edges and make the blades thin. The one and only function of the blades in producing energy is to shovel the wind from one vector direction to another. I will accept no disagreements or other viewpoints on this.