MITCHAA,
I think that in the event of a major main gearbox failure leading to sudden loss of torque on both engines the result would be both engines trying overspeed.
The overspeed control system would shut down the first engine to hit the target speed (120% N2???) then it would self disable and the second engine would runaway.
This would probably result in an engine explosion as the free turbine gave out.
An indication of what happens when the overspeed system is inoperative is given by the Helikopter service Super Puma that crashed in the mid nineties.
In this case the drive to the main gearbox failed and the thrashing driveshaft cut the cables to the overspeed sensors. The engine FCU control system could not react fast enough and the engine exploded.
None of the above is intended to suggest that this is what may have befallen the S76 in question.