Fuel Burn - lots of different factors need consideration.
Fuel Burn questions are very individual & boat specific & are also very speed related. Many factors go into the number. Hull shape, displacement, prismatic coefficient, drag due to shaping, depth of the hull, hull design - like - planning hull vs semi displacement hull vs full displacement hull, all effect fuel burn.
You also need to know the waterline length of the boat that the engines are in as the relationship of the speed length ratio has more effect on fuel burn that you may realize.
Take the square root of the water line length multiplied by the Speed length ratio you choose to cruse at.: 1.0; 1.1; 1.2; 1.3; & top hull speed 1.34. to determine the S/L ratio cruising speed.
Please refer to Robert Beebe's book for that info in more detail.
--- the lower S/L ratio gives better fuel economy by a large % factor.
You say 8.0 to 8.5 knots - but if your at the Max hull speed for the water line length of your example boat at 8.0 knots you can have a huge increase in fuel burn just to go that 1/2 knot faster !
A S/L ratio economy example that is good at 1.2 S/L Ratio cruising speed for local scooting around cruising speed & 1.1 or 1.0 S/L would be good for long distance runs.
So be advised other considerations need to be put into the equation. For Instance, There is sometimes more under water drag with a twin engine boat than with a single. Twin engines combined fuel burn will often be more than a single engine vessel at same comparable specs.
However, if your just looking for a fuel burn number example for these engines, I had a DD 6 V 71 turbo on a 46 footer with a 39 foot water line length single & it burned about 5.2 GPH at 8 knots at 1800 RPM. The DD's are a two stroke design & in my experience they definitely are more thirsty than a regular 4 stroke engine of same displacement & power output.
I am
guessing for comparison the same equivalent boat with 4 stroke would be around about 3.8 to 4.2 GPH. under same conditions.
I hope that info helps. There are many other factors as well, like are you towing your dink, etc.. I am sure there are more guys here on TF that can give their examples & input as well.
Good Luck.
Alfa Mike