I admit I'm a little out of my depth with this question, but unlike an airplane, both engines in a twin helicopter are coupled by the same load. So if one engine starts surging and generally misbehaving then what does the FADEC do to the other engine good engine that doesn't know about it's twin brother's indecisiveness at providing the necessary power to the MGB?
What would the engine indicators be showing in such a condition on the 135?
The good engine will take over. F. e. if you fly with 2 x 55% TQ, when in OEI you will get ~110% TQ on the good engine unless you lower the collective ( powerlever ) and get the approbiate indications on your engine instruments, split needles ( N2, TQ etc )
skadi