Writing is a lonely occupation, and sharing the challenges and risks with another author can sound more appealing than going it alone. Unfortunately, finding and working effectively with a collaborator can much more difficult than achieving solo success. The risk could be:
Collaboration is most successful when writers have complementary skills. The challenge is in finding a partner who complements your skills and who needs the skills that you bring to the party.
Collaboration can lead to personality clashes.
To work effectively with another writer, you need to be confident in both yourself and your collaborator. You also need the ability to divide responsibilities, review each other's work, and make decisions about character, plot, scenes, etc. without letting egos get in the way.
Collaboration requires a double dose of self-discipline.
What if one of you proves to more efficient than the other? The history of collaboration is littered with manuscripts that never got finished because Writer A had to wait for Writer B to do his share of the work.
Collaboration can raise tricky legal issues.
Let's say that you and your collaborator have been working on a book together. Your collaborator doesn't hold up her end of the bargain, and you're forced to complete the manuscript on your own.
What happens if you sell that manuscript? Your collaborator may demand 50% of the advance and royalties--or, worse yet, your collaborator may refuse to accept the publisher's offer.