Consider 1 important factor: Pounds/horsepower.
A 311 weighs 7400 lbs. with stock 365s (730hp) is 10.13 lbs/hp.
A 271 weighs 5250 lbs. with stock 7.4 L (310hp) is 16.9 lbs/hp.

I don't believe a boat with twins burns twice the gas a single engine boat does...With the same hull. The lower the ratio, the more efficient the boat will be not bringing hull design into the equation. Air entrapment will make a hull more efficient but more horsepower will too. If you compare a 271 Fastec with a heavier 311 SR1 then the power to weight ratio changes, but not by that much. I wouldn't be making my decision on gas mileage, but rather cockpit layout, condition and reliability. I've owned lots of great boats and they've all been different. Buy a boat that suits your needs, not how much gas it burns.

Lbs/HP is for speed comparison, not fuel efficiency. There's some correlation when you're comparing apples to apples: say a 311 with 330s vs 365s or a 271 with the 310 vs 365. But fuel burn is not rocket science: double the volume, double the burn.

I've owned a 272 and a 311.Both big block Bravo drive boats.I don't boat often but when I go I will typically do a day trip of 90 to 180 miles.Usually cruise at 3000 rpm, carb is just at the point before the secondary barrels open, and running Bravo 4 blade props, 26 pitch. Boat is going about 42 mph, a little trim and no tab.Both boats would consistently get 1.5 to 1.6 mpg cruising like this.

I agree. And there is also a relation between horsepower and fuel burn. Typically an engine burns 10 gallon per 100hp per hour.
The difficult point is how much hp is required to push the boats in this example to a certain speed. WOT is an easy calculation, but below that it is difficult. Also the rpm's don't relate lineair to fuel burn, the produced torque at that specific rpm's gives horsepower and fuel consumption.

In the above given example, in my opinion the single engine of an 271 needs to produce more hp than each engine of the 311 does presumed both boats are at the same speed. For that reason the 311 would not burn double what the 271 burns.
Double fuel burn would more or less apply at twice the weight with comparable hulls, or a higher speed of the 311.

Just my thoughts

If comparing twins vs single in the same hull, twins will always be more thirsty and less efficient.In addition to the engine friction of the second motor, the twin engine boat is dragging that second drive through the water.The hull is also dragging more due to greater weight.I think that buying the boat you need for the conditions you boat in should be a bigger concern than fuel economy.