According to the method the crossover phase of the friction clutches is performed by defining a linear torque profile for the first friction clutch having a given initial value and a zero final value, a linear torque profile for the second friction clutch having a zero initial value and a given final value, and a linear torque profile for the engine having a given initial value and a given final value. The final value of the torque profile of the second friction clutch is calculated, the initial value of the engine torque being known, by determining a final value of the longitudinal acceleration of the vehicle equal to a given percentage of the initial value of the longitudinal acceleration of the vehicle. The final value of the engine torque profile is calculated, the final value of the torque profile of the second friction clutch being known, on the basis of the final value determined for the longitudinal acceleration of the vehicle. The initial value of the torque profile of the first friction clutch is chosen so as to be greater than a minimum value calculated by determining the condition of synchronism of the driving shaft and of the first primary shaft.