Stuart,
You are correct in your observation, and correct in your understanding that connecting both carb's ports is superfluous.
It was easy to do both with rubber hoses and a tee-fitting; and, in doing it this way, the unused no-threaded port/pipe does not need to be blocked off (I'm lazy and I easily talked myself into thinking that it looks OK/better).
When I plumbed a set of dual stock Rochesters, I only plumbed one carb. The vacuum port on the other Rochester was easily sealed with a brass screw cap; and, I was able to use the stock all-metal vacuum line to one carb.
Tim
