Barit1, I've just read your post again, and I quote: "........governors, one on each shaft, and either one may "take charge" to keep things within limits."
I think I misunderstood your terminology, I know them as 'speed probes', not governors so apologies for my misinterpretation.
There's also the mechanical TOS (Turbine Overspeed) system which is a cable and centrifugal pawl device that cuts the fuel supply in the event of a shaft failure or excessive overspeed.
