It depends on the diameters and the load if a Savonius rotor works as a brake. A Savonius rotor has an optimum tip speed ratio of about 1 and a H-Darrieus rotor has an optimum tip speed ratio of about 4. So if the diameter of the H-Darrieus rotor is four times the diameter of the Savonius rotor, both rotors run on the optimum tip speed ratio if the load is correct. However, a twin Savonius rotor with a diameter of only 1/4 of that of the H-Darrieus rotor will have a torque level which probably isn't large enough to bridge the part of the Cq-lambda curve of the H-Darrieus rotor for which the Cq-value is negative. So generally one takes larger twin Savonius rotors and then the optimum lambda for both rotors is no longer realised for the same rotational speed. Then the Savonius rotors will work as a brake if the H-Darrieus rotor runs above a certain tip speed ratio. But the ratio of the rotor diameters can be chosen such that the Savonius rotor is running just unloaded if the H-Darrieus rotor is running at the optimum tip speed ratio. You should know the Cp-lamba or Cq-lambda curves of both the H-Darrieus rotor and the twin Savonius rotors to find the ratio of the rotor diameters for which this is realised. For this ratio, the total height of the upper and the lower Savonius rotor must be chosen that large that the torque level of both Savonius rotors together is large enough to bridge the part of the Cq-lambda curve of the H-Darrieus rotor for which the Cq-value is negative.