In a case where two or more people are inextricably linked to the creation of a work, there would be a joint ownership of copyright. In order for this to occur, one of the two following criteria have to be met.
Either, the contribution from each party must be inseparable, or the individual contributions combine to form a complete creation that is not comprised of distinct elements.
For example, if a song was written where one person wrote the lyrics and another person wrote the music, each person would have their own copyright ownership of their contribution.*
However if two people wrote an instrumental piece of music, a joint ownership of copyright would occur.
When someone wishes to use a work that has a joint ownership of copyright, permission from each owner of its copyright is always required. Should all permissions not be given, then an infringement of copyright could still occur.
In any collaboration an agreement can be made where one party could claim ownership of copyright. Each collaborator must give their explicit consent for this to happen. Such an agreement may wish to be made in order to make giving permissions easier in the future.
A joint ownership of copyright can – like any copyright ownership – be sold, transferred, inherited etc. Once again, all collaborators in a joint ownership of copyright must agree to the transaction.
* If someone wanted to use the lyrics without the music, or vice versa, then only the permission of that particular author would be needed. But if the complete song was wished to be used, then joint permission would still be needed.