I guess at present you have one lever sending only one signal (one volt level). However with two pedals you get the possibility of conflicting signals.
What command does each pedal send? I guess one is throttle? Does it automatically spring up when you remove your foot?
What pedal position causes forward and reverse motor direction? Does it automatically spring to a certain position when you remove your foot?
To drive backwards, is the speed unchanging or adjustable? Is there a switch somewhere in your present thumb lever, to choose forward/reverse?
Do the pedals permit you to press both at the same time? Does one pedal assert priority over the other? With three wires there may be simple electrical methods to replicate all functions.