Again, the way this system works, in the lead unit, the controller is sensing only 1 speed- the tach motor, and that sets the overall voltage to BOTH motors in that unit. If the tach reader truck slips- then the other truck may grip, but then also not be given more voltage unless the throttle is raised even higher. Not ideal but just how it works.
When you have a trailing slave board unit in PS2- that is an "analog" board- so called because the signals it's getting from the lead unit are direct motor and light outputs and then just "buffered" by the board. The slave unit board has more or less a similar circuit as the lead motor drive unit using a relay and 2 FETs. This way, when it duplicates the lead unit PWM, the resulting PWM or voltage to the trailing engine motors is identical to the lead unit. In practice, I find that the trailing powered unit actually has slightly less pull with the slave board- but that is just how the system works.
But again, ALL of this is ONLY effective when the lead unit tachometer motor and truck is NOT slipping!!! Again, the system only really works when the most important truck and motor- the one with the tach, is not slipping.
This same problem can be found in countless locos- Example the MTH Turbotrain. On that engine, due to the shell and clearance, the rear truck in that only has one drive axle and traction tires and is known for slipping, meanwhile the front truck is a powered 4 wheel truck.
