Actuators under J1939 should run at the same speed if you tell it to run at the same speed.   The resolution of the signal is 5% of measured duty cycle on the motor per bit.  However, this percentage is dependent upon the load, so if one actuator is seeing a load slightly higher than the other, speed will change.  Same goes with the slow one, if it has a heavier load on it, it will run slower, but give you the same percentage of speed (duty cycle).  Since the signal is 5 bits long, you control between 0 and 25% of duty cycle, as a ratio of 0 to 100% of speed.