I do not use a double screen board for combining. I use newspaper.
I think that a common misconception is that the queen pheromone wafts through the air and completely fills the entire space much like I can smell my wife's great cooking down in the kitchen even though I am upstairs.
For bees, the pheromone is transmitted not in an aerosol fashion but rather molecule by molecule by her attendants. Attendants physically touch her with their antennae and feet and I suppose other parts and then as they move around the colony, her pheromone smell molecules go with them and are thus transferred to other bees. But when the double screen is in place, the attendants cannot get even a single molecule to those bees upstairs because of the gap between the two pieces of screen. They might stick their proboscis or antenna up through the first screen, but it never reaches the bees up above. As a result, the upstairs bees soon feel as though they are queenless because of the sudden loss of pheromone molecules in their neighborhood. This is the principle that comes into effect when using a Snelgrove board to make extra queens as I am doing myself this week: Put nurse bees with eggs/larvae/pollen upstairs. Put queen with capped brood/open comb and honey downstairs. Separate with Snelgrove double screen board. Girls up top think they are queenless so start making emergency queen cells. As we speak, I have a queen maker colony doing just this in my back yard. They have been separated with a double screen Snelgrove board for 8 days. Top box has 10 queen cells ready to be moved into mating nucs. If the pheromone was aerosolized, the upstairs nurse bees would NOT be making emergency queen cells. But they are.