Two stepper motors at different speeds using potentiometer and Ardunio
Why not using arduino libraries? For instance this StepperDriver library.
Usually all stepper drivers like A4988, DRV8825 and industrial drivers use STEP (or pulse) DIR (direction) and EN (enable).
It should not be too complicate to connect the potentiometer readings.
Check out the libraries exemples about speed here
Hope that's helpful.