hello Retroplayer
i guess this would not be a good idea but if your willing to try,
Each h-bridge has a forward and reverse signal, btw. 10 is forward, 01 is reverse, 00 is off, and 11 is a no-no (no braking.)
if you do decide to use pwm to control the speed your set up one way to do it is by controlling one of the pin
for example if you want to go forward, you have to pwm the first pin.
for braking, you could for instant if you go forward pwm the second pin but use only 50 that way you have total braking.