For dual rotor, the disc area is generally considered to be that which is not overlapped. Solidity is per disc. The disc area is used to define the cross section of the outflow (stream tube)- where for given thrust level, less disc area requires higher flow speed.
These layouts are not efficient, which is why 95%+ of working helis are of a conventional layout. Flettner layouts are typically quite slow, where drag is a real problem. Weight can be also- so they are typically built using very low mass blades with absolutely minimal complexity as regards cyclic control (ie. servo tabs to twist the blades- which is less possible with robust/stiff blades- this doesn’t scale up well with size or speed).
But, they can be very stable- as a function of some very complex aero/rotary dynamics. And that helps for something like the Kmax- where you throw a massive turbine in the thing, keep its mass very low, live with low speed and no internal payload- but achieve stable hovers and a high payload to gross weight ratio.
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