Agreed, if you're going to run multiple carbs you need extensive tuning, a 400 only needs technically 650cfm, Quads ran 700-850 max claimed cfm, actual was probably less , you might be able to go with 2 400cfm's or similar but just putting larger carbs on may do nothing but puke your motor unless it's built for it, extensive head and intake work, chamber work, valves, ignition, etc all to flow match what you want to pull in, you're going to punch it and your motor is going to cough.

I've seen so many guys put on duals and a tunnel ram only to that set up is only good for 2500-7000rpm, rest of the time no hp no torque. Bigger carbs or multiple carbs delivers more air & fuel to the engine. All components must be properly matched in order for performance gains to be achieved. If the motor can't suck in the cfm delivered it will lose power, that was the main issue with the RAV, the heads and intake were so huge the air/fuel mixture didn't know which way to go, and zero Vac".

It's really just a way for some companies to sell you 2 carbs, now if you have a roots blower, maybe, still, extensive tuning.