This new application note will show you how to use a dual 50Volt 5Amp H Bridge Motor Drive "MD22" from http://www.robot-electronics.co.uk/htm/md22tech.htm
You can use 2 potentiometers to control 2 different motors.
For each potentiometer/motor:
At middle position the motor stop.
Turn it on the right and the motor turn forward.
Turn it on left and the motor turn backward.
The Debug window will show you (with a text) how each motor turn (forward or backward) and also give you a value of speed (0 - 127).
All is here: http://www.lextronic.fr/rayon.php?id=167
See in the application note #18.