They're claiming a 30% reduction in fuel consumption in cruise on one engine, which is nothing to sneeze at. This seems reasonable given the SFC characteristics of turboshaft engines at part load operation. Shutting down one of the engines during cruise will also reduce the number of hours accumulated on each engine.
But there are also the issues of starting a turboshaft engine on short notice. And the increased fatigue placed on the individual gearbox input drives from operating them at higher loads.