I concur that idling one engine for extra endurance is false logic. All the fuel into the idling engine goes to waste running the compressor and accessories and certainly with a Turmo that's a lot of fuel wasted, especially as the alternators run off the MGB anyway.
Single engined flight can be more efficient, certainly for endurance but possibly not for a transit, subject to the prevailing wind (the reduced max IAS results in a longer airborne time).