Reselect both copies made in this step, open the Pathfinder panel (Window > Pathfinder) and click the Minus Front button. Select the resulting shape, set the fill color at black (R=0 G=0 B=0) and lower its Opacity to 5%. Make sure that this thin, new path stays selected, open the Graphic Styles panel (Window > Graphic Styles) and simply click on that New Graphci Style button to save a new graphic style.