Electronic switch

Using Arduino General Electronics
1

Ok so im running a polulu qik 2s9v1, and i want my motors to get more power, so i had an idea to change the motor part of it. the power coming out of the motor driver will trip a switch and turn on a circuit which consists of a motor and and a battery, removing any chance of the motor over powering the driver.

What sort of switch would be best for this?

2

If you need to control second motor speed, better to find another more powerful driver, than try to amplify current.
What is the voltage/current or power of the bigger motor?