Currently 2 different vocal credits added at the same time (one AR) appear as "a and b vocals:" under the recording (release). But the same credits added sepately (multiple ARs) appear as "a vocals:" and "b vocals". It makes sense to make each vocal credit a separate AR and only display them combined on the webpage (when it makes sense).

Example recording with combined and separate credits: http://musicbrainz.org/recording/c5870765-0aeb-49dc-bd6d-5aae117706df