You should theoretically be able to trash the engine with excessive fuel if operated manually, since there is no supervision of the engine limits.
With full FADEC fail of one engine, you will also lose the TQ and TOT figures (its a function of where the data routes through the FADEC black boxes, whereas the N1 is direct from the motor) and also the FLI needle since it has lost 2 of 3 inputs. You can only match engines by N1 figures. This is quite fiddly, especially with stiff twist grips. An easy procedure starts with noting your take off TQ and N1 figures.
With a FADEC freeze at 20-25% TQ leave it alone as you should have sufficient power for climb, and be able to descend.
On finals, as power demand builds up thru 7.5 FLI on the good engine, set manual engine to the hover N1 figure and leave alone. This will give you both engines for hover landing. Lowering the collective, wind off power to avoid Nr rise.
Hope this helps.......from an EC135/145 beefer.