4-barrel carburetors are flow rated at 1.5" of Hg (That's 20.4" of H2O) pressure drop across the carburetor with the throttles held wide open. If you have an engine that needs 750CFM say at 6500RPM, and you have a 750CFM carburetor on the engine your manifold vacuum at 6500 would be 1.5". Assuming atmospheric pressure is 30.00" of Hg, your manifold pressure would be 28.5" (30.00" - 1.5"= 28.5"). To make the most power we want to run as close to 0.0 manifold vacuum (30.0" of manifold pressure) as possible. Superchargers actually put positive pressure in the intake to make their power.
Now if you installed 2, 750's on the same engine, the engine will still need 750CFM at 6500. That is all the airflow you will get through the carburetors (carbs are not superchargers and dont automatically fill the engine with more air!). Each carburetor will be passing 375CFM of air. However the manifold vacuum will only be .375 of Hg under the wide open throttle plates. (There is a fairly straightforward formula for this, but it is not easily written in a manner that can be posted here.)
If the manifold vacuum is .375 of Hg, then the manifold pressure is 30.00-.375= 29.625 of Hg. The difference between the manifold pressure at WOT between the dual-quad application (29.625) and the single quad application (28.5) is 1.125 of Hg. If you divide this difference (1.125) by the manifold pressure of the single quad (28.5) you get an answer of 0.039 that in percentage is 3.9%.
What this all means is the manifold charge density is 3.9% greater with a dual quad system as with a single quad system in this example. If the single 750 equipped engine made 450 HP, the dual quad engine would make 3.9% (or 17.5HP) more..467.5HP. This proves out very closely on engine dynamometer tests. Now the difference is not always 3.9%, it varies with the relative size comparisons. The bigger the engine and the higher the RPM, the more the difference becomes.
One very important point is that there is a minimum manifold vacuum at WOT at which the booster venturii will start flowing the proper ratio of fuel. For this reason, it is always better to use 2, vacuum secondary carburetors (I prefer Holleys because it is easy to run an equalizing vacuum line between their secondary throttle diaphragms) than 2 mechanical secondary carburetors on the street. With the vacuum secondaries, only the primary throttles open up when you go to WOT. The secondary throttles open only as airflow through the primaries reaches a point where the WOT vacuum will create enough airflow to cause a good booster signal on the secondary throttle side.
Two 600s work great on an application like this (especially with the vacuum secondary equalizing kit that Holley sells). Many people buy the 2, 465CFM carb set-up from Summit. This is a terrible set-up as you are getting mechanical secondaries without a secondary accelerator pump. For a customer who insisted on it, I have made 2, 600CFM Holleys work on a 283 Chevy in a 63 Impala with a tunnel ram and a Powerglide. The engine had headers and the cam was a 204/214 @.050 with 420/443 on a 112LSA. It drove like a dream and got plenty of looks. It didnt smoke the tires on a WOT stab from a dead stop but it didnt bog either. It ran just like it had a single 4-barrel on it. I admit I have an advantage; I was a development engineer at Holley back in the 70s so I know a few little tricks, but it is not difficult. Jetting was box stock. I agree that a single four-barrel is easier to work with, and with all the different sizes there are a lot of choices, but if you need that last bit of power, 2,4s can get it for you and they look impressive. Just keep in mind that the power increase going to dual quads is pretty minimal unless you are over 500CID. Hope this helps!