how to use decoder and motor drivers to power individual motors at 12V?
No, you are describing logic-level devices which can only switch logic level signals. They are not capable of switching 12 V motor currents.

If you have only one motor driver then you have to switch the motors. This may be done with relays or, if they are DC motors and only run one direction then a transistor or MOSFET could be used to enable each motor by connecting one of its terminals to the 12 V power supply positive or common. Meanwhile the PWM from the motor controller could be connected to the other terminal of each motor.