One or two ducts to center vent?

[dmjlambert]

In the LMC catalog there is an option for buying plugs to replace one of the hoses to the A/C center vent (plugs are item 16). And in the Brothers catalog they have a special adapter for the center vent that only has a fitting for one hose, and they are claiming on their website and in a video that output of the center vent is increased by 40% by using their one-hose adapter because when using 2 hoses there is a canceling effect. They are saying "A similar modification was illustrated by GM back in 1972 to fix the problem of poor center vent volume." Does anybody have any experience with this one-hose-to-the-center-vent idea vs. the factory design?