A 2 × 2 bbl. manifold has to be common plenum, there's no way to divide front 3 cylinders from rear 3 without a 12 port head.
A 3 × 2 bbl. can be 3 separate manifolds (cylinders 1-2, 3-4 and 5-6) or plenum (typically progressive linkage to run on the center carb first).
How are they different?
the 3 × 2 carburetors have to be much larger.
