Amon just doesn't want to fuck Dhalia. He wants to make Rea get used to him being with other women. She is young and is not as mentally tough as her mother.
He needs to make Rea realize the sexual dynamic between them. So by doing this, using Dhalia as a front for teaching and letting Rea watch him have sex with another woman will make her get used to it.
He just needs to be careful. Luckily for him, Rea is almost under his control. Once he takes her, she, too, won't be able to go back and become addicted to him, just like her mother.
Rea looks at Dhalia suspended in the air with all four of her limbs tied and spread wide apart, waiting to be used by Amon. She looks at Amon and wonders what he will do with Dhalia.
"Let's get started. Are you ready?" Amon asks Rea. Rea looks at Dhalia and back at Amon before nodding her head.
Since Amon wants her to look, she will do as her daddy says.
"What about you?" Amon questions Dhalia.
