In the unlikely event that both channels failed the metering valve will freeze in the position it was in when the both FADECS agreed there was a failure. The system then reverts to a third buck-up system, the EBCAU which operates independantly from the two FADECs. The back-up system is set to give you aprox 102% NR and will get you to where you can land with no problems but you are supposed to avoid abrupt manoeuvres.
There is no manual connection between the twistgrip and the engine as this is a true FADEC. The twistgrip rotation simply triggers the TWT GRP warning light, then it contacts the idle speed microswitch commanding the 67% idle.
WRL