Figured it out. After you beat the doll girl and complete the chapter you can go to the giant hole and find the key in one of the pillars, the key to the hidden door behind the wall scroll in the right room. Once you get the gem you can use the machine to go visit the past of the doll girl apparently and some guy who dies to the green and blue doll girls. Then, replay chapter 2-2 and when you get to the doll girl again you'll fight the true final boss of Chapter 2, magic spamming green and blue doll girl. Beat the boss and the doll girl joins the love shack. For the dance dance monster girl all you have to do is dance and beat her, I'm assuming with a high dance percentage for her to join you.