The middle of the twentieth-century featured several famous papers with two authors. For example, Eilenberg and Mac Lane's papers introducing categories and Eilenberg-MacLane spaces appeared in 1945. The Feit-Thompson Odd Order Theorem appeared in 1962. Atiyah and Singer published their index theorem in 1963.
I can't think of any important papers with two or more authors before the Eilenberg-Mac Lane collaboration, which could just be a lacuna in my historical knowledge. My question is: what are the first math papers with two or more authors? (A subsidiary question is: why were collaborations so rare before that?)