From a fuel burn rate there is a cross point at which 2 engines are more efficient than one as pointed out above. In a PC you would have to be going prob. less than 1000 rpms on one engine, which is a bout 4-5 knots to make it more efficient. Moorings used to deliver the PC this way some 6000 miles and many days and alternating engines. Now they ship them on a freighter and drop them off in Sir Francis Drake channel.

As for my use I always run 2 engines as I like the added maneuverability it gives me as well as the speed. Also it evens the hours out on each engine and creates hot water for both hull cabins. Given my crusiing speed, one engine would be too much strain and pull, not even sure one would get that high of speed. I have some performance graphs on my boat that I can share. The Moorings(R&C builders) have them on the most all of their PCs, especially the newer ones