Alex, This house is a custom design for a specific lot on Lake Chelan. The concept was for an outdoor living room and an indoor living room with the atrium available to both allowing for those days when the wind is blowing. This way you can be outdoors, with the view on the windy days, which are many.
As a custom home, the plan is zoned for the owners side, and the guests side, both sharing the common area in the center.
This idea could be modified, I suppose, but to be a functioning duplex would require some rethinking. I'm not sure the common area works that well for two separate occupants.
But, it is designed for waterfront, so some of the ideas could be incorporated into a duplex.