During my many hours on the subway each week, I often stare at print ads for novels. Mind you, these are almost always GC (publisher of Nicholas Sparks and other zillionnaire best-sellers). The lurid ad text makes you yearn to read the thing on your Kindle before you get to work. I mention the publisher to make clear that these are not literary masterpieces. They are hugely viable commercial monsters. They are closer to TV shows than books.
Many of these ads are for two-authored novels. Think of it. Two people writing the same novel. The concept fascinates me. It really is more like the procedure for a screenplay.
I try to imagine the roles of these two authors. One guy writes while the other guy sleeps, so it's a 24-hour factory? That would explain the proliferation of such books. One guy plans the story and the other guy writes it? One guy writes the dialog and the other guy sews it together with "he said," "she remarked," "the girl blubbered"? One guy writes a draft in longhand and the other guy types and revises? One guy writes while the other guy does a Starbucks run?
For someone who (like most novelists, I feel safe in guessing) thinks of writing as blissfully solitary work, the collaborative novel seems as ineffectual as two people driving the same car at the same time.