How to use shoulder button mobility

We are programming on a VEX joystick and we are trying to figure out how to use the 5u and 5d buttons for mobility (forward and backward exclusively) during the manual period, is there any possible way that can work or do we need to just use the joystick(s) to drive. We are planning on using the 3/4 joystick for lateral movements.

Similar to the code in this thread.
https://vexforum.com/t/coding-issues/43508/1

  if( vexRT[Btn6U] == 1 ) {
    // run motors forward/up
  }
  else 
  if( vexRT[Btn6D] == 1 ) {
    // run motors backwards/down
  }
  else {
    // stop motors
  }