1) The PF525 offers velocity vector control only, and although it has good torque control, it does not have an actual torque regulator in that tiny little package. So you can't do a second drive as a torque follower using that series of drive; there is no torque control mode in a 520 series drive.
2) The Autotune parameter, P040, ALWAYS shows the initialized status, whether or not the autotune function has been performed. If you are unsure of whether or not it was done, just do it again, it can't harm anything. If you can uncouple the load from the motor and perform a rotating autotune, that is better than the static tune, but if not, the static tune is the bare minimum that must be performed.
3) When you use Vector Control of any sort with a PF525, it IS a velocity PID loop control, you don't need to add another one. Double check that you are in fact using the Vector Control mode, either Sensorless Vector (SVC, P039 = 1, the factory default), which doesn't use the encoder, or Flux Vector that does (P039 = 4). Since you have the encoder anyway, I would use the Flux Vector control. If P039 is set to 0 because someone thought that was easier and they didn't do an Autotune, that may be your problem right there... this is DEFINITELY a Vector Control application.
If those checks and any changes result in no improvement, either your mechanical situation needs attention of your motor is just too small for the load. The idea of a slave drive is probably the best approach, but you will have to change to a PowerFlex 753 for both motors.