In theory, adding venturi area increases peak power slightly... if the new setup has the same or better fuel distribution as the original manifold (developed from decades of experience).
Dual carburetors are only useful in a siamese engine operating on both* (not progressive linkage), which means throttle response must be somewhat rich to be acceptable in traffic, and adjustments are... shall we say, critical vs. OEM.

* there's no way to use 1 carburetor each on the 1-2-3 and 4-5-6 cylinders because the 3 & 4 cylinders use the same port.

If I wanted a bit more power I would adapt a smog-era air cleaner assembly and duct cold air from the grill.