Hi
Technically it is not good to double the load of your circuit, you may burn it. another important issue is what you are activating with this motor you got, it is suitable for a toy car, but it may not be suitable for moving a heavy object. if you want to apply the ouputs to another motor you can apply a current amplifier which can be done by a follower emitter circuit. About doubling the voltage you need further analysis of your circuit, the characteristics of the IC and your motor.
Sal