Try this:
Declare nice_motor as a variable byte
Then state that:
nice_motor = 127 + (127*button1) - (127*button2)
And of course, serout nice_motor to the desired PWM pin
What will happen:
If button 1 is pressed, the motor will go full forward, if button 0 is pressed, it'll go full reverse, and if you press them both (who would do that?
), nothing happens.