What was the first SciFi book or story co-written by 2 or more authors before 1957?
(1957 is simply because I'm aware of one written in that year, so that is a definite early mark).
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The Space Merchants by Frederik Pohl and Cyril M. Kornbluth saw daylight in 1952. It came out as a book in 1953.
When Worlds Collide by Philip Wylie and Edwin Balmer dates to 1933.
The Skylark of Space by Edward E. "Doc" Smith was written between 1915 and 1921, with the writing help of Lee Hawkins Garby. It was serialized in Amazing Stories in 1928. Book publication was however delayed, until 1946.
J-H Rosny, one of the early science-fiction authors, was actually a pseudonymous co-authorship between the two Belgian brothers J.-H. Rosny aîné (Joseph Henri Honoré Boex) and J.-H. Rosny jeune (Séraphin Justin François Boex).
Les Xipéhuz évoque la rencontre des hommes avec des extraterrestres d'origine minérale. Faute de pouvoir les comprendre, les hommes finissent par les massacrer...
Les Xipéhuz relates an encounter between men and extraterrestrials of mineral origin. Unable to understand them, the men end up massacring them ...
[N]omadic tribes in the land that was to become Mesopotamia, the cradle of mankind, are confronted with mysterious creatures shaped like translucent cones of living matter fraught with magnetic energy. Their sudden appearance on Earth and proliferation threatens the very existence of the human species. The Xipéhuz are living minerals and so fundamentally different from men that no communication can ever be established with them. Contrary to Wells's Martians, who are shown as a further development in man's evolution, the Xipéhuz represent a form of life for which there is no precedent or conceivable model in the human mind. Eventually the invaders are annihilated due to the ingenuity of an exceptional man...
Which certainly qualifies as Science Fiction in my judgment.