How to have multiple digital outputs for the same pin on Arduino Uno and Simulink?
You cannot try and have two things controlling one output in two different ways. It just makes no sense.  Like trying to have two people steer a car and have one go left and the other right.

I don't quite know what you're trying to do, but under normal usage using a motor driver requires 3 pins (for example the common L239D). Two define the direction and a third (the EN pin on the L239D) controls the speed with PWM.