Would a digital motor be too simple? how about using it possibly as an electronic gearing system, sample an analogue voltage from a sensor, say with a range of 0-1.25V, then send the digital data to the DAC and provide a signal out proportional to the sampled voltage. The ratio could then be user controlled and an exact speed reading for both motors would also be obtainable, fed out to an LED array or something
Best i could come up with in 2 minutes
Frank