if you've checked the motor itself and found no shorted A1 to A2 and the cables checked out ok then the problem is in the controller itself. This code is triggered by high stall current across the transistors, most cases is from stalling the motor.

this stall code is a dead giveaway when you check the controller code history. I always found it useful to know if the operators were pushing loads or not.

and no this truck is a single motor drive system, the 2nd controller is for hydraulics, it is known as a premium hydraulic control system.
(at least thats what yale used to call it years ago when they first started using the SX controllers in the hyd systems)

in any case though since your not finding any shorted cables and the motor checks ok then you will have to either have this box repaired or replace it with a new one. There are no servicable parts inside this box for the tech. I do believe flight systems has reman box's available.