Bill. Good question. You’ll only need one carb to supply the vacuum signal to the distributor. Hooking up both carbs does not change or increase the vacuum amount as that is driven by the air flow speed within the carbs at those particular ports. And because you are dealing with two carbs, the air flow speed within each does cut in half. In this case, there is no doubling effect of the vacuum signal when hooking up both carbs to the distributor. The reduced air speed is still the controlling factor. If using a stock distributor, then modifying the spring tension in the vacuum advance chamber can allow for some additional advance during cruise situations and restore the overall ignition timing attributes back to a more optimum design. Although rarer than hens teeth now, a factory dual quad distributor would be the easy fix.

Wayne. By the book and for the combination you ask about, there should be no vacuum at idle going to the distributor assuming the engine is in a good condition and properly tuned. As the engine wears or other problems come to the forefront which in turn causes the throttle blades to be opened more in which to compensate, then the potential is there for the blades to be positioned above the ported vacuum hole in the carb body thus allowing a given amount of vacuum signal to be present at the distributor at idle. Hence the need to disconnect the distributor vacuum line when setting the initial or 'at idle' ignition timing.

Timmy (via Slick56) brings up a good point regarding the secondary balance connection when non-dual quad Teapots are being used on a 2X4 setup. While I’ve been retrofitting the single four barrel Teapots with an equalizer port at the right rear of the carbs similar to what the factory did on the List# 1268, 1434, and 1437 carbs, hooking together both the distributor ports at the front of the carbs effectively does the same thing.

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Another option for the secondary equalizer or balance line between non-dual quad Teapot carbs is to simply put a hose connection in each secondary diaphragm cover and connect the two covers with a length of hose similar to what was done with the 427 Ford dual quad setups. There are obviously several different ways to accomplish the secondary balance between the Holley Teapot carbs if actually wanting to do this on converted single four barrel carbs.