After my refit I did sea trials operating with one engine only or with both. I was surprised, initially, that for a given speed the fuel consumption was effectively identical: the single engine at higher rpm and load to give the same speed used the same fuel as running both engines. Not that surprising when you think about it: the fuel burn is the HP required to balance the hull resistance. Now, the idle prop will have some extra drag, but that effect was lost in the rounding/measurement accuracy. This was at a number of points between 5 and 8 knots.
So the only reason to run on one engine (other than a breakdown) is to go slower than is possible with twins, perhaps to really max out the range.
Twins do have an overhead for maintenance, as well as original install if you are doing that. But the extra maneuverability is worth it.