How about simply forgetting all about the two stages in the PT6, or even the whole engine for that matter: on the King Air, the torque is measured in the (propeller reduction) gearbox. What sits behind the gearbox is of little or no concern.
When the propeller levers are retarded, the governor goes to an overspeed sensing condition and allows oil pressure to be reduced from the hub to coarsen the blade angles. While this causes the propeller to slow down, the blade AoA increases, and so does the torque in the gearbox.
