Welcome to ribnet!
I can be absolutely no use here with a direct answer, but there have been numerous fuel consumption threads in the past. Have a search through the forum (the search engine is pretty good) ansd see what drops out.
As you are presumably thinking of going twin, be aware of the "perennial discussion" surrounding twin engines & economy..... There are some fairly strong views on the matter!
Having said all that, "accurate" fuel economy is a bit of a difficult one for boats - for example if you motored upriver all day, you would be drinking more per mile than going downriver with the "road" pushing you along too. Weight & it;s distribution, engine trim, hull shape & driving style are also big influences on fuel consumption. A good rule of thumb seems to be about 1L/ nautical mile as a going in point, regardless whether you have a 3L V8 on a 26' hull or a 2 cyl 25Hp on a 16' ......