Bring water and milk to boil in medium saucepan over high heat. Stir in macaroni and reduce heat to medium-low. Cook, stirring frequently, until macaroni is soft (slightly past al dente), 6 to 8 minutes. Watch closely as it easily boils over Add American cheese, mustard, cayenne and cheese powser and cook, stirring constantly, until cheese is completely melted, about 1 minute. Off heat, stir in cheddar until evenly distributed but not melted. Cover saucepan and let stand for 5 minutes. Stir macaroni until sauce is smooth (sauce may look loose but will thicken as it cools). Season pepper to taste.