A FADEC digital control has the mathmetical sophistication to use integrators as delta3 describes, so it can be truly "isochronus" - constant rpm - and still be very stable.
Unless you have an engine ARINC crosstalk failure. Then, they both go to good old proportional mode and the Nr droops in a similar fashion. (And, 30sec power is armed due to each FADEC not knowing if the other engine has died)
Hoss