From an attorney…
If two authors work together to write a book, they become joint owners in the copyright to the book. Unless the writing was separated in some easily discernable way, both authors own 50% of the work and each have all of the rights that a single author would have. If both authors contributed to every chapter and worked together to decide on wording, there is no way to differentiate between their work and therefore no method to divide the copyright.
On the other hand, if each author wrote certain chapters of the book and those chapters could be separated without destroying the work, then there might be a way to divide the copyright. Typically, that is not the case. The applicable federal law states that “a ‘joint work’ is a work prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole.”
Co-authorship can be problematic because both creators have the right to commercialize and monetize the product and can easily (and frequently do) interfere with each other’s efforts if there is a lack of cooperation.
When co-authorship is contemplated, it is recommended that they enter into a written assignment of the copyright to avoid joint ownership of the copyright. One of the authors can assign his rights to the other author in exchange for compensation, recognition, and other negotiated terms.
If the co-authors cannot agree on one owner, they can both assign the copyright to a company in which they both have an ownership interest. The company, such as a limited liability company, should be governed by a written agreement between the owners. That agreement should include the rights and obligations of the respective members, how to deal with contingencies such as the death of one of the owners, buyout provisions by one owner of the other’s interest, as well as other operational matters.
When a creative work is owned by only one person or one entity, there is a clearer direction for commercialization of the work and there are far fewer disputes over the respective rights and obligations of the parties.
From an attorney…