manual color mixing

i have been playing around with a dual color mixing extruder... what i want now is to build a manual electronics control board which would take the motor pulses from the ramps board and drive two pololus in a certain ratio . The ratio can be determined by a simple knob... So when i turn the knob the ratio changes in real time....

any ideas how to implement this? one way is to use a frequency divider.. maybe it is not a correct way ... any better ideas?