What is the second battery connected to?
In a typical installation there may be one battery dedicated to starting the motor and a second battery dedicated to providing electricity to everything else in the boat.
If the main battery were to fail such that it can not start the motor the second house battery would be available to start the motor. Normally using the house battery only.
If the house battery was unable to start the motor then the two batteries would be connected in parallel using the both setting on the battery switch. Hoping that the two batteries together can start the motor.
Getting both batteries charged when the motor is running is another subject altogether. Many different ways to do that.
