Everyone seems to have a different opinion on settings for the Airflow. I have my bottom screen on the middle fixing and the top screen almost as low as it will go. That works best for me (with Aprilia mirrors & extenders) the air can come over the top and hit my helmet as well as the air around the sides.
Yes, I believe the principle is the greater the overlap of the screens the greater effect it has, the air gets pushed up between the screens and alters the airflow around the sides. (This could be b******s of course but someone will correct me
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Try all the settings you can until you find the one best suits you.