I see no merit in the balance tube - also I have never seen dual carbs with the balance tube. I actually think that it would be detrimental to the function of the carby (why are the runners individual in the first place? and why would you want to balance the signal after the throttle plates? when #1 is on intake stroke #4 is on power stroke.)
There is merit however in drawing a combined vacuum signal - it will probably assist in giving a consistent vac sig at all RPM (this is mostly relevant if you are running quad throttle bodies AND a MAP based reference to an EFI computer).