I’m honestly of the opinion that it’s only Damien inside the body. Celine mentioned sending the viewer back, and Damien mentioned that you needed to let him in–nowhere was it mentioned that Celine would be going along, she only mentioned being able to send you back. Damien himself says this–about himself. He does not mention “let us in,” simply, “let me in.”
I think it’s more likely she transferred a little bit of her power to Damien when she sent him back, as a form of help. In my mind, the bit of her powers that she gave him when she sent him and the viewer back were what adds the red touch to Dark. Not Celine herself. It just doesn’t make sense to have both in the body. And then as soon as Damien was in the body, he took it over and booted out the viewer.