Prep a 9x9 pan, coat generously with butter or cooking spray.
In a medium pot, melt 2 tablespoons of butter over low heat. Once melted, add in 6 cups of mini marshmallows and stir until completely melted and smooth. Stir in the Rice Krispies Cereal.
Pour mixture into prepped pan. With greased hands or a utensil, quickly and gently press down into an even layer.
In a medium pot, melt 1 tablespoon of butter over low heat. Once melted, add in 3 cups of mini marshmallows and stir until completely melted and smooth. Stir in the Fruity Pebbles Cereal.
Spread the Fruity Pebbles mixture evenly over the Rice Krispies layer, pressing down gently so the layers stick together.
Let the treats sit for about 30 minutes to firm up.