Apart But A Part

Four main characters, duplicated and intertwined, all with their own subplots, in two differing realities (A Part), existing simultaneously yet never interacting with each other (Apart). One reality revolves around a screenwriter/director writing a screenplay about a novelist, the other a novelist writing a book about a screenwriter/director. Which one is real and which one is imagined, or do they both exist. - or not exist at all?