If a copyright work created by the cooperation of two or more persons constitutes an inseparable whole, all co-authors of this work shall have joint copyright in it and the use of such work is decided jointly by all co-authors.5
In the case of co-authored work, it is advisable to establish the respective contributions contractually beforehand, or at least as soon as they are produced, if the contributions of all contributing authors are not proportionally equal.
Co-authorship is only present if the co-authorship existed at the time of the creation of the copyright work and the work is therefore an inseparable whole. Co-authorship is not present if each author is working independently, and the work of one has inspired the work of the other.6
5 Source: 12. Copyright and Related Rights Act, Article
6 Source: The Decision for Responsibility, av. Sašo Urukalo, publishers:DOS and MM, 2010