How much is too much dual quad

It depends on a few things. Type of carburetor. Size of carburetor. Style of linkage. Progressive or straight. Two Thermoquads, for instance, although unorthodox, the engine could probably stand two 850s. Since the secondaries on the Thermoquad are regulated by engine vacuum, that means the engine will only use however much of that total 850 CFM each that it needs. Two 850 Holleys would drown it, while two 850 Thermoquads properly tuned would peel up the asphalt. Same with the Carter or Edelbrock AVS style carburetor, since they also have 100% vacuum regulated secondaries, like the Thermoquad. The Rochester Quadrajet is another example of a vacuum regulated secondary carburetor. Also, regarding the linkage, a progressive style linkage will allow the carburetors to come in more controlled a little at a time, instead of all at once like straight linkage. So unless you nail down which carburetor and linkage you plan to use, the answer is pretty all encompassing.