Flying a twin with one engine at idle  

Yes. An engine at idle is just wasting fuel. You need to shut one down to gain anything worthwhile.  
do the guy who said fly high you are correct if the aircraft can do it efficiently. I flew the dauphin. It did very well above 9000.  The agusta 109 would actually do worse above around 7000 due to loss of efficiency of the rotor system.  Fuel burn went down but speed dropped way down.