Place 8 graham crackers in bottom of 9 in. x 13 in. baking dish. Mix vanilla pudding and 3 cups skim milk, let sit 2 minutes. Gently fold in Cool Whip. Pour half of vanilla pudding on graham crackers. Top with another 8 graham crackers. Pour remaining vanilla pudding over graham crackers. Top with last 8 graham crackers. Mix the chocolate pudding mix with 1 1/2 cups skim milk, let sit 2 minutes. Spread over graham crackers. Let it sit overnight in the refrigerator to soften the graham crackers.