@mycheatingdad,
You can't save their marriage. It's not you job, and you are far too close to both of them to act as referee. Please don't get involved in their marriage this way. It will only make things more painful for all.
Tell them both you love them. It is appropriate for you to express your feelings as in "I feel sad when you hurt each other". It is not good for you to take sides or be in a position to "save" them. They are going to have to figure it out for themselves.
You are the child. They are the parents. You can't change roles.