Originally Posted by
Checkboard
... of course slowing to best range speed with both engines running is more efficient than doing so with just one running.
0n a conventional twin {engine on each wing) possibly, due to additional drag generated from counteracting asymmetrical thrust, on a centerline twin, very likely not, depending on the brake specific fuel consumption curves for the engines, prop efficiency curves and such. If you need 150 hp to maintain best range airspeed, and you have 2, 250 Hp engines, it may be more efficient to have one engine running at 150 hp, than two at 75 hp. Note that the Rutan Voyager cruised on the rear engine with the front engine shut down and feathered.