Surely the only reason the engine instrument readings were similar when the #1 PL had been retarded was because the miss identification had already occurred and the serviceable (#1) engine had been shut down?
Look at the time indicated by the red line I've added - just before the #1 is slotted
EDIT: Can't get img hosting to work, look at approx. time 53:20
With the exception of Np, all the other readings would have been similar at this point in time - possibly adding to confusion?