Include both last names. In a parenthetical citation, place an ampersand (&) between them.
A few years later, British cosmologists (Collins & Hawking, 1973) discussed theflatness problem in the context of the anthropic principle.
If the names of the authors are in the text, use the wordand instead of an ampersand.
A few years later, the British cosmologists Collins and Hawking (1973) discussed theflatness problem in the context of the anthropic principle.
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