From the Super Mario Wiki
The Mandibug Stack is a pair of enemies appearing in Super Mario Galaxy and its sequel. It consists of a large Mandibug with a smaller Mandibug on its back. It appears in Super Mario Galaxy in the Honeyhive Galaxy mission "Trouble on the Tower", and Super Mario Galaxy 2 in the Puzzle Plank Galaxy mission "The Puzzling Picture Block" (after the puzzle on the final planet is solved), serving as a sort of mini-boss in both games. To defeat the Mandibug Stack, Mario or Luigi must first defeat the smaller Mandibug on top with a Ground Pound. Upon doing so, the larger Mandibug will become angry and approach Mario faster than a regular Mandibug. Defeating the larger one reveals the Power Star.