Singles that are a collaboration between two artists should have both mentioned on the release group.
Currently only the first artist mentioned on the release group is shown.
For example:
http://critiquebrainz.musicbrainz.org/release-group/d7b72274-fc87-475c-9430-8ee41093f288

This should have zedd and foxes listed on the release and links back to the artists.
http://critiquebrainz.musicbrainz.org/artist/19fccfa3-84ad-42b2-b52d-d5d9a5a6046e
http://critiquebrainz.musicbrainz.org/artist/56c4b861-0922-4c3a-a9b9-3bfcb00f8274