No. Unless there is an agreement to the contrary, each co-author has an undivided interest in the entire work, so each can license rights to it independently of the other. There is a duty to share royalties. When you talk about "the agreed upon royalties" that makes me think perhaps you have an agreement that both must consent to any license and defining how royalties will be shared and in what ratio. Alternatively, you might have an agreement that you each share the royalties and what the total royalty rate will be, in which case the shares will probably be held to be equal shares. The point is that you can contract away your undivided interest or your right to be independent. Call me if you need help with any copyright questions like this one. There is a good detailed answer online at http://www.publaw.com/joint.html from a publishing law specialist attorney. Wow! I hope I never have to get that specialized.