Melt the butter and marshmallows in a large pot together. Keep an eye on it, while stirring occasionally.
In a large bowl, combine the two cereals. Gently mix to integrate them together.
When your butter and marshmallows have melted, remove from heat and quickly add in the cereal. Work quickly to mix these together.
Pat the mixture into a greased cookie sheet or jelly roll pan. Allow to cool.
Cut with sharp knife or pizza cutter. (I like to remove it from the pan first, in order to not scratch my pans.)