I am not sure I have started this well. If they spin the same direction, they will create the static pressure in two different zones in the cage. Alternate the spin for each, and they will not create the same level of static pressure, but will at least be creating it in the same zone inside the cage. Since flipping the curve makes for better flow, makes me suspect enough to ask the question. Again, the cage is going to function better without cavitation and interruption of flow. Admittedly, my experience is not dust cyclones, but in alarm horns. The principles are common though.