I have yet to see a boat test for a twin engine boat that had peak mileage much over 2 mpg. I have seen a number of boat tests with singles that got peak mileage of over 3 mpg. Small blocks or big blocks, the results were similar. You just can't push two drives through the water, and expect to get great mileage. When you add an extra engine, you are adding over 1000 pounds to the boat, so that contributes to even more drag.
I will add that some sort of cobbled up rich running carbureted big block single isn't going to do well for mileage either. Go EFI, and have it properly tuned.