The only problem I see is cycles. You're doubling the cycles on one engine each flight. Well, each flight over a certain duration anyway. You wouldn't be shutting one down for a 25 minute Medevac, but on longer offshore flights it could be beneficial. But they need to get engine manufacturers onboard with cycles, otherwise the engine will cycle out way too soon, affecting DOC's!
I wonder if a more commercially realistic approach would be to have one engine at idle? Then it could quickly be brought back online if needed in an emergency.