First, it must be determined if the work is a "joint work." If two or more writers co-write all the material together (as opposed to one writer writing one section, and the other person writing the remainder), and assuming that there has not been any agreement that it will NOT be considered a joint work, then as a general rule, it will be considered a "joint work." If it is, legally, a joint work, then as a general rule either co-author has the right to adapt, and to issue NON-exclusive licenses to third parties for the adapted work, but must account to his/her co-owner for the co-owner's share of any income which the adapting party receives from third parties. This is a complicated legal area, and the above information is only intended to provide a general overview, and should not be considered legal advice for any particular situation. (There may be factors present which would be exceptions to the general rules stated above). It would be wise to have a qualified copyright attorney in your area review your particular situation and make recommendations to you accordingly.
Answered on Nov 17th, 2011 at 3:34 PM