BB2, you can often get a better idea about powerboat fuel consumption
once you home in on a specific boat and specific engines. A 35' Carver with gas engines, for example, doesn't necessarily compare closely with a 40' Carver with diesels. But once you know more, you can sometimes find somebody who can give you more accurate info.
Usually fuel consumption is reported in gallons per hour (GPH) at a given cruising RPMs, but if you've got apples and oranges, it usually helps to compute and compare nautical miles per gallon (NMPG)... IF/IF you know approximate speed. And that's sometimes a bit more difficult to come by.
In the meantime, starting with what you've given, and assuming a 35' gas Carver, you could maybe guess approximately 40 GPH total (20 GPH per hour per engine) at approx. 20 kts. (That's a VERY rough guess., but based slightly on our previous 33' gas boat.)
42' boat with planing hull and twin 450-hp diesels cruises at about 20-22 kts and uses about 26 GPH total (approx. .75 NMPG)... or putters along just under displacement
speed at about 7 kts using I think something like 3 GPH total (~2.3 NMPG). But that won't help you much if you're looking at gas.