Because of the balance port in the manifold, all carbs affect all cylinders to some degree - as vette says, the front carb would have it's biggest affect on the front three cylinders but will affect the rears too, just to a lesser degree.
I believe this is why both jets should be down the same distance from the bridge and both carbs should have the same air flow at idle and more importantly, off idle. Then, if everything else is correct, you theoretically have the same a/f mix and volume in all cylinders = smooth running engine.