[spacemanspif]

The quadrajet that came factory on my Monte Carlo used a similar setup with 2 small primaries that were good for easy cruising and 2 BIG secondaries for when your foot got heavy. Others with better tuned engines than mine have reported getting close to 25mpg highway. Best I ever saw was around 18.

I don't think multi-carb intakes work in the same manner. Usually a dual quad setup has a runner dedicated to each port on the carbs. However, a stock 6-pack intake might use the 2nd and 3rd carbs in progression to keep up with needed power...not sure how exactly the worked though.