When more than one artist performs on a release, how they're credited depends on whether they're a permanent group or a one-off collaboration. Both Apple and Spotify converge on the same underlying rule.
2.4.1 The compound artist rule
Each artist field must contain only one artist name. Listing two collaborating artists as "Artist A & Artist B" in a single field is a compound artist error on both platforms — Spotify's metadata guide defines this explicitly: when two or more individual artists are listed in one field, the individual artists won't be correctly identified and the content won't link to the correct artist pages.
2.4.2 Correct treatment of a genuine collaboration
Each collaborating artist must be listed as a separate Primary artist in a separate field. "Paul McCartney + Stevie Wonder" is two artist entries, not one combined field.
2.4.3 Bands and permanent duos are not compound artists
A band or duo that always performs and is credited together — Simon & Garfunkel, Brooks & Dunn — is listed as a single artist. This is the opposite situation from 2.4.1: the group name IS the artist name.
2.4.4 Short-term named collaborations
For short-term pairings between two otherwise-independent artists (the Apple guide's own example: Bill Evans & Jim Hall), credit them as two separate artists at both release and track level — do not credit the pairing as a single combined entity.
Source: MadDesk article 2.4 (2-4)