This video gives a good analysis of twin V6 200s vs a V10 400 on same boat same conditions. Can skip to the analysis about 30min in. Chart below is how they scored it but it depends on what you prioritize for how you run your boat. Fuel economy overall is better for the single but at higher speeds it favored the twin setup. For someone who cruises at 40mph they would be burning less fuel than the single at that speed (for this boat and motor combo).

For their performance data the single is on the left and the twins on the right.
