This is very interesting. I'm just adding up the benefits.
It overhangs (or could overhang) the house significantly preventing sunlight from getting in if you are in a hot climate.
The roof is exposed to less sunlight, not exposed to hail, generally stays dry. If the 'top roof' leaks, it is easy to detect and fix but not of an immediate consequence.
This is a radiant barrier that isn't in an attic and allows the space between the heat absorbing top roof to be cooled by even a light breeze.
In snow or hail climates you could adjust the overhang to be ideal and it would block most snow.
Solar panels on this overhead roof would mean no penetrations into the structure.
I'm not seeing any large disadvantages to this as long as it isn't overhanging sunlight in the winter in an area that needs such an overhang. Cost, space, and aesthetics are factors but so is every design decision in homebuilding.
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