You need manifold vacuum, not necessarily connection to the carbs or to either carb. If you have non-progressive (center squirt) linkage and are trying to obtain some sort of throttle blade sync between the two carbs, you need one of the air horn sync devices used on old side draft sports cars and individual bike carbs. In other words something that will "read" vacuum above the blades
All that will do, tho, is maybe improve off idle/ low speed/ street manners
