Helooo!!! I dont speak english that well… so sorry if I had any mistake.
You're talking about the polyamorous route right?
Because there are a lot of different routes depending on what you choose.
Anyway,For the polyamorous end you must first, inagar more and when you get to the shadow garden go back, then do the vanilla route, grabbing the book. Then make the proposal and answer shadow correctly. Inside the closet, stay but don't take off your clasp. Then beat both of them in the cards and earn their confidence in the food. To finally touch the shelf and give them the blueberry juice to drink.
And that's how you get it!!! I hope it works for you! ;3