The solenoid bleeds air to under the hood. If you hook the hose to the left one, the air will come out of the right one. If you hook the hose to the one on the right, the air will come out through the one on the left. Does that make a difference? It is possible that the valve is not symetric, and will allow a little more flow in one direction than the other, but I don't think that matters, either, until you get to the point where one stock solenoid is not big enough. I know there are "high flow" solenoids, but I don't know how you tell when you need one.