You may find this style guide helpful, especially “Artist credits should generally follow the actual credit used on the release / track, including the join phrases” and “If the release has no join phrase for multiple collaborating artists, but just lists them all separated by whitespace, use the default join phrases (an ampersand between the last two artists, and a comma between the others).”
I think the litmus for making a single artist vs multiple artists out of a duo like that, credited by name, is “are they usually credited together / do they usually operate as a group” (having trouble finding that but it’s in the guidelines somewhere), which in this case I would say is a yes. Compare to husband and wife duo Robin & Linda Williams.
Be sure you’re not confusing the artist credit, which is generally what’s printed as the track artist, with the performers, though. Performers can be sourced from anywhere, as long as there’s convincing evidence, and should be added with relationships, preferably to each track. See this release, where the artist credit is The Hopeful Gospel Quartet but Robin and Linda Williams are credited as performers (and note they are credited individually, just like you would credit separate members of a string quartet).
If they all sing in each song, you should add them as performers to each song. Although also note the release-level relationships on the release above, which is what you do if it isn’t clear which specific tracks that artist performed on.