Right now it's in specific areas of the game, so it's a little difficult to give you a straight forward answer. For some it's combined after choosing breast/ass expansion, for other it's an actual path itself.
The characters you're referring to are the two main characters, they currently don't yet have an hourglass option but are planned to. I was talking more specifically about other characters, for example Nylah, the character you interact with in the introduction. She has hourglass in the intro itself and in her later story.