It seems to me that the claims about power efficiency are optimistic or probably just wrong. The tether between the kites will have to add a force in the kite’s ‘drag’ direction. My guess is that you end up approximating more traditional drag mode concepts in efficiency, except now you don’t get the hit in propeller conversion efficiancy which is possibly bad.
One thought is that the kites need to fly on a long tether because the path chosen will quite quickly lead to a large cosine loss. This will probably favor super liggtweight slow flying kites to efficient rigid wing kites/blades.
Another thought is whether this is the easiest approach if you were able to actually keep two kites airborne consistently.
Third thought: The kites need to be well synchronized in order for them both to fly at optimal aerodynamic conditions. If one of the kites slow down, it would get tugged backwards by the other (strong) kite. From the very brief study I have done so far I would expect radio communications would be a practical requirement for this type of rig (most designs would use a radio, but most don’t depend on them). Radio is a big issue when you get to the point where you want to fly above people.
I think this is a good idea in the sense that it introduces some new mechanics to kite rig design