ok now i understand. basically you want to make the second potentiometer as a knob you would control this making is almost like the knob example like the one that the arduino IDE gave in its servo sample. Am i correct about this?
well if this what you one i am gonna give you some hints...
1)set the middle value of both pot.
2)determine which part of the value will make the motor turn clockwise or anticlockwise
3) compare the value of both pot
4) the difference between the pot could be use as to determine the speed
5) decide on what type of breaking you want to use
6) if both pot is the same value then the motor should stop.
7) if you could use PID control /fuzzy control would be great.
please keep me updated with your progress..