So what I can remember is at the beginning of the book, the other woman kinda forced H to have threesome with one other woman. H and OW were in entertainment world? H was convinced he couldn't live without OW (or that she had his secret? I can't remember clearly).
And I'm not sure about this one, but it seemed that the heroine was the OW's assistant.
A Kiss for the Camera by Olivia Jaymes has some of the same elements. The H and the OW are famous. He is an actor and I think the OW is one too. The h is an author. I think there was a set-up for it to look like he was cheating on the OW with two women. But if I remember correctly, the OW wasn't a real relationship, it was something arranged by their agents. ??