Spray 13x9-inch pan with cooking spray. In very large bowl, stir together both cereals; set aside.
In 4-quart Dutch oven or saucepan, melt butter and peanut butter over medium heat. Add marshmallows; cook and stir until completely melted and mixture is smooth.
Pour marshmallow mixture over cereals. Add peanuts; stir until cereal and peanuts are evenly coated. Using sprayed back of spoon or hands, press mixture firmly in pan. Cool about 1 hour.
For bars, cut into 6 rows by 4 rows.