Hi @Spinorose -
Unfortunately that driver board doesn’t seem to accept “servo style” PWM signal, and so will not be compatible with a PixHawk - not without potentially modifying some firmware parameters - however the 20,000 hz required may be higher than the system will be able to produce.
Generally it’s a bad idea to try and parallel driver boards, as slight differences in their output voltage will lead to currents flowing where they shouldn’t be, damaging components. Is more documentation or a link to the hardware you’re trying to use available?