Another set of rotor and blades attached to an existing windmill for more energy generation. The size of the other set of blades will be smaller, as shown in the cover image.
Two sets of rotors will generate more electricity. The quantity would not double since the other set would be smaller (between half to two-thirds of the size of the original set of blades).
A smaller set of blades can be used because if they are of the same size, they will block the air and only the rotor in the front will work.
The wind does not flow with the same velocity continuously. The velocity increases and decreases. When the velocity decreases, it might not be sufficient to move the blades that are made assuming a higher wind velocity. In this case, even when the velocity is less, the smaller rotor can work and generate electricity.
Basically, below a certain threshold, the smaller rotor will work, and above it, the original will work. If it is observed that the rotation of the smaller rotor hinders the air current hitting the bigger rotor, when the bigger rotor starts moving, the smaller could collapse into a bud. It will open only when the bigger blades have come to a stop.
I could not find multiple concentric rotors on a windmill. The closest thing I found was this (twin windmills) - two different sets of blades and rotors next to each other on a single shaft.
Hindrance in the flow of wind to the bigger rotor is the only problem I could foresee (which has been addressed). Anything else?