G'day Paco,
Kinda depends on the aircraft your considering ....
Offshore ...... 212 ??? I guess depends on the aircraft configuration but work out the specific consumption .... prob no point winding one of the donks down as things can possibly get worse ... beep Nr down to 97 or so max Tq for cruise and cross fingers ....
Believe RAF used ferry the Pumas with one donk at idle to get a significantly better specific consumption and have done the same myself so I know that works!
Onshore ...well I guess you land when the pucker factor gets excessive!
I remember (when on the way to the Antarctic a long time ago) seeing an RAAF Orion appear out of the stratus trying madly to get the 2 shutdown engines online ... so I guess they have an SOP to shut down donks to increase sfc while on patrol.
Cheers