As far as I know both motors are controlled by the same stepper driver, you switched the controller board so we assume the new board does not have the same problem.
And you switched the motors so you think the motors are good. And still you having problems I only see one remaining thing and that you are overloading the stepper driver.
What if.... one of the motors it just a little bad and draws more current then the other motor? and combining them will result in a load the stepper driver can't handle?
I don't know your board but it will not differ much from a standard ramps I think. On the ramps board we have 2 x 4 pins in parallel controlled by one driver.
You could measure the 2 coils in your stepper motor, if you download the data sheet it will become clear what to measure.
