Many years ago I had a 36' boat with twin 383s. It burned about 4 gph per engiine at 1800 RPM. On a plane (20mph) at 3400 it used about*12 gph per engine.
No, I never did it. Gas was a whopping 60 cents per gallon so big cost items were elsewhere. I'm not a fan of one engine at a time due to*prop rotation issues on other engine. But, different strokes ---