Co-written

Collaborative fiction is a form of writing by a group of three or more authors who share creative control of a story.

Collaborative fiction can occur for commercial gain, as part of education, or recreationally – many collaboratively written works have been the subject of a large degree of academic research.

Our overall process changes from book to book. Usually while I'm working on another project, Mary will come up with an idea, run it by me for my input, then get on with plotting and research. Nothing's ever set in concrete. No two books follow quite the same procedure.

A collaborative author may focus on a specific protagonist or character in the narrative thread, and then pass the story to another writer for further additions or a change in focus to a different protagonist. Alternatively, authors might write the text for their own particular subplot within an overall narrative, in which case one author may have the responsibility of integrating the story as a whole. In Italy, various groups of authors have developed more advanced methods of interaction and production

The methods used by commercial collaborative writers vary tremendously. When beginning writing the short story 'the toy mill' Karl Schroeder and David Nickle began by writing alternating sentences, whereas when English authors Terry Pratchett and Neil Gaiman wrote Good Omens they largely wrote separate plotlines and then collaborated much more heavily when revising the manuscript.

The collaboration may be very limited indeed, when John Green and David Levithan wrote Will Grayson, Will Grayson the only plot point they decided on was that two characters would meet at some point in the novel and that their meeting would have a tremendous effect on their lives. After this decision, they separately wrote the first three chapters for their half and then shared them with each other. After sharing, they then "knew immediately it was going to work", as stated by Levithan.

...
Wikipedia