This will happen on level 43. You start with 4 power buttons only. You need to start the conveyor belt closest to you, then start the one behind it, making sure to line up the 2 impostor penguins next to each other as they move down the conveyor belt. Then, just before the pass in front of the mallet, power the mallet on to hit them both off at the same time. Note there seems to be a split second delay from when you power the mallet on, to when it actually swings, so it may take a few attempts. Make sure to turn the mallet off again after you hit the 2 impostor penguins.