These guys are comprehensive:
2.6.2. Collaborative artists should be split into their individual names except when the combined name was intended and can be verified as the group name (for example Simon & Garfunkel), legal songwriting entity (for example Lennon-McCartney), or production partnership (for example, Stock, Aitken & Waterman). Verification can be found, for example, in official external websites, biographies, and discographies.
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