Relative gas consumption from 2 vs 1 engine

How would the total gas consumption compare between 1 big engine vs 2 smaller, as in 1- 20 hp vs 2-10 hp or 1-100 hp vs 2-50 hp?

 

To general a question. What you need to do is find the BSFC (Brake Specific Fuel Consumption) for each engine. It measures pounds of fuel per hour per horsepower. A good number is around 0.50, plus or minus.

 

The question is simple enough, but the answer isn't because there are too many factors. Specific fuel consumption is a secret that most manufacturers do not want to share with you, except the diesel guys like Cummins. A modern diesel uses approx. 205 grams per hour for each HP, that's 30-50% better than any gasoline engine. The figure is always given for the rpm with max. torque.
Two engines have a bit more friction than one, but smaller engines with carbs control their mixture better at part throttle. Injection engines do not have that problem. Two props can be more efficient that one, and they steer better, but the main reason to use two engines always was and is the reliability.
I helped a guy yesterday who had a belt guide roller broken on his 320 HP Merc. No belt tension, so no cooling, no power steering, no alternator. Two smaller engines would have been better.