How to syncronize two motors with one pot?
You are trying to synchronize the motors on the basis of the input to their velocity loop.  I doubt this will get you the results you desire.

What you need is often called Electronic Gearing.  The Wikipedia page on Motion Control describes it briefly.  Put simply, Electronic Gearing requires a position control loop on one motor, slaving it to another on the basis of position.  The master motor runs in a speed control loop and the slave motor runs in a position control loop using the position of the master motor as input.  As a result, the slave motor appears to track the position of the first motor independent of speed or load.

You didn't post a link to the datasheet for your motor controllers, but I'd start by determining if they support it natively; many do.