On this page of our Psychonauts 2 guide you'll learn where to find the Half-A-Minds located in the third level of the game - Lady Luctopus' Casino.
Location: Hollis' Mind.
This secret can be found immediately after entering Hollis' mind. All you need to do is to jump onto the black platform with a purple pattern.
You'll find this secret on top of a moving platform in the casino, during the Maternity sequence.