The efficiency of a turbine engine is driven mostly by the percent of its thermodynamic power that it is using. The hotter it is running the more efficient it is. A good modern turbine should burn about .4 to .45 pounds of fuel per hour for each horsepower while cruising at 75% power. If you put two engines into the helicopter, each as powerful as the one original, they will be operating at about 40% power each, and the fuel consumption could be about .5 or .55 pounds per hour per horsepower. That means that a powerful twin might burn 25% more fuel than a single with the same power. A twin with two small engines could be the same as a single with one big one, of course.
This factor is the reason why helicopters with lots of excess power are relatively big fuel hogs, and have comparatively less range (Wessex, anyone?).
It is also a paradox that the excess power forces the aircraft to carry more fuel, which robs it of performance (that 25% extra fuel to go the same distance means less power can be used for payload or engine out performance).
Putting one engine in idle is almost a sure way to waste fuel. The back engine is in its most inefficient state (some fuel flow and no power) and the increased efficiency of the up engine never makes that up. However, if you have the nerve 9and are willing to see exactly how fast you can get a restart) you could shut one down. This has been done when range was needed and the alternative is a ditch, but it is tempting Murphy.