I havent' seen one yet, but if you're handy enough with electronics, you could do it with a couple of reed switches (or other passive or active wheel speed sensors) a quad op-amp chip, integrating the pulses from the two wheels' speed sensors separately, then comparing the voltages, then using the difference to add or subtract from the throttle's actual voltage.
Might not want to ever actually add to it, since going faster might cause slip on the one wheel that still has traction, but slowing down the faster wheel (presumably the motorized one) ought to be ok.
Might take five opamps rather than four, depending on how you mix the signals to get the final throttle input to the controller, but four ought to do it. Sorry I don't have an actual circuit, but using basic op-amp circuits available at a few of the "learning electronics" websites would probably work, once you work out the passive component values needed for whatever voltages you want to run this all at.