Songs written by more than one person are known as “joint works”.
The creators of joint works (for example songs written by a band) are equal co-owners of the
copyright unless they have agreed to the contrary.
Whoever contributed to the composition of the work has an equal claim to the entire copyright. The copyright is not divided up according to who did what. Each joint author owns the copyright and each has rights to use or license the work as long they split any money earned with the other joint owners. One of the joint owners cannot prevent another from using the copyrighted work.
For example, when one person writes the lyrics and the other composes the music, both are
considered contributors to a “joint work” if the intention was to combine those elements into a single work – the completed song.
Unless there is a written agreement to the contrary, both contributors would have an undivided ownership interest in the entire copyright and would have the right to perform, reproduce or license the work (or any art of it) without the others’ consent, provided each copyright owner is paid a share of any revenue.