On DC-10 and MD-11 there are official procedures for 2-eng. ferry
flights in case of technical problems to reposition the aircraft.
No passengers allowed and many procedures/restrictions to be
followed. The company I worked with was rather generous applying 2-eng. ferry on the MD-11 until they had a fire warning
on one of the life engines on such an occasion. It was a false warning but after that event things were handeled more re-
strictive.
I don't think you'll be able to save fuel on a passenger jet by
shutting down engine(s) in cruise-you'll be slower and will have
to fly lower-just check respective tables for all eng. cruise against
eng. inop. cruise. Not to mention possible legal requirements.
And some engines don't like beeing windmilled for
hours (lubrication problems).
Cheers, jojodel