This is just wrong. As mentioned in http://forums.musicbrainz.org/viewtopic.php?pid=21512#p21512 a lot of choirs have both (and a lot of choral releases have both credits) and they're different positions.

Want to update it as per http://wiki.musicbrainz.org/User:Reosarevok/Conductor_Relationship_Type and also make it clearer in http://wiki.musicbrainz.org/User:Reosarevok/Chorus_Master_Relationship_Type