I have been reading “John Ciardi: a Biography” by Edward M. Civelli. Although Ciardi is best-known as a poet, he did write one science fiction short story - The Hypnoglyph - co-authored A Grossery of Limericks with Isaac Asimov, and was generally interested in the field. In particular, Civelli tells us that in 1953:
Ciardi’s chief occupation that year was Twayne, where he was developing science fiction titles from his home office and working on a new concept called Twayne Triplets, which was conceived as a series of books, each about a “hypothetical (but thoroughly possible) planet different from ours.” Once the hypothetical planet had been given its essential characteristics the data was given to three established science fiction writers, each of whom was to write his own story fitting the “facts”. The resulting hardcover triplet would therefore be one book with three independently written stories...
By 27 March 1953, Ciardi had eight of these books in various stages of development and production - with twenty four authors, contracts, and manuscripts to juggle and balance.
It sounds an interesting proposal. However, Ciardi was unable to come up with the financing for the project, “and so the deal fell through”.
Looking through isfdb, I see that there are two “Twayne Triplets” listed: Witches Three containing an essay by Ciardi, together with works by Fritz Leiber, James Blish, and Fletcher Pratt, and The Petrified Planet, containing works by Fletcher Pratt, H. Beam Piper, and Judith Merril. I have not found munch information about Witches Three, but The Petrified Planet fits exactly the criteria above, the details of the planet being devised by Dr John D. Clark of “Ignition” fame.
Assuming that Witches Three was one of the books that Ciardi proposed, do we know anything about the remaining six? Even through the deal fell through, presumably the author list had been settled - it would be interesting to know who they were!
Andrew has discovered another proposed triplet, that would have contained Question & Answer / Planet of No Return by Poul Anderson, Sucker Bait by Isaac Asimov, and a third story by either Virgina Kidd or James Blish. Further more it seems that A Case of Conscience by Blish was intended to be published as a Twayme Triplet (either this one or another).
A blog posting by Ricard Horton also reveals that:
... stories written for Twayne Triplets include “A Case of Conscience” and “Get Out of My Sky” by James Blish, “Second Landing” by Murray Leinster, “Sucker Bait” by Isaac Asimov, “Question and Answer” (aka No World of Their Own) by Poul Anderson, and “First Cycle” by H. Beam Piper (later completed by Michael Kurland).
Brian Kunde suggests that The Great Fetish (1978), by L. Sprague de Camp was also originally intended to be part of a Twayne Triplet.)
This is confirmed in L. Sprague de Camp's autobiography (see the comments on the blog posting).