In MusicBrainzTerminology, a split release is a Release with tracks by more than one Artist, often only two. The cover of the release often has no given name, but only features the artist names. There exists an official guideline outlining how such releases should be titled. There is no equivalent guideline for the release artists, but the most common usage is "Artist 1" / "Artist 2" (etc.) - individual artist credits separated by the join phrase " / " (with spaces!).
To add an album like this, add a new release. In release information, add the first artist credits as usual, then click the double arrow to the right of the 'Artist' field. Select 'Add Artist Credit +', and add the second artist. Remember to change the join phrase to " / " (with spaces).