South of the 2nd elder you will find a house with some bushes by the ledge. When you walk over to the ledge Everett says he wishes he had a satellite map of the area. This is a hint to use the Eyeball Costumes' exploration ability. Pulling the camera back it will make plain a path between 2 bushes on the right.
Go down the path and fall down to the platform below. Go into the cave. The achievement will pop then. Inside the cave are 2 characters from https://www.trueachievements.com/Stacking-xbox-360.htm
, the next Double Fine game.