Recently a new technology of wind turbines has been introduced which is going to have a remarkable share in the market in the close future. This wind turbine has two set of blades. So it is more efficient for collecting energy from wind in comparison to a single rotor wind turbine. This paper investigates the dynamic behaviour of the dual and single rotor wind turbines under unbalance transient conditions. To reach to this stage the models of mechanical and electrical components which exist in PSCAD/MTDC library have been used and combined together according to the configuration of the single and dual rotor systems. Both wind turbines are actuated and monitored from three aspects. Firstly, a step change in wind speed is applied which leads to a change in mechanical torque. Secondly, voltage sag is considered at the generator terminal which causes electromagnetic torque shock. Third, the amount of energies which is released by the mechanical systems after network frequency drop, are compared with each other to evaluate which one can support the frequency better. In all calculations the stream tube effect has been entered to the simulations. Different variables of wind turbines are evaluated and the operations of the rotors are compared.