Vestas surely has tested the efficiency of these multi-rotor devices.
Beside that, close to each other Makani turbines did not seem to have been a problem.
But it may be that the structure is very heavy while some mass is gained on the small rotors. And the whole structure must turn when wind direction changes.
Perhaps an arrangement of the turbines in a circle like this can also improve the density, harnessing a whole wind frontal airspace like multi-rotor frames intend to do, but without such heavy structures.