Spray bottom and sides of 13x9-inch pan with cooking spray or grease with shortening. In large bowl, mix both cereals and peanuts.
In 3-quart saucepan, heat peanut butter, marshmallows and milk over low heat, stirring occasionally, until peanut butter and marshmallows are melted and mixture is smooth.
Pour peanut butter mixture over cereal mixture; stir to coat well. Press mixture in greased pan. Cool at least 30 minutes before serving. Cut into bars.