The plot of causing two cards to fuse into one was published by Norm Houghton as “Stranger of Another Color” in Ibidem, No. 5, Apr. 1956, p. 19. Houghton had a blue-backed card fuse to the face of a red-backed selection. No signatures were involved.
Roughly eight months later, a routine by Mark Weston, “Fantasy with Two Faces”, appeared in Abracadabra, Vol. 22 No. 568, Dec. 15, 1956, p. 346. This is the second seminal item in the development of the fusion plot. The performer takes two seemingly random cards from the deck. He places them into an envelope, from which he proposes to make them disappear. But things go wrong—only the faces vanish, leaving a single card made of the two backs fused together. And the two faces are found in the middle of the deck, also fused together as a single card. To right the situation, the double-backed card and double-faced card are put into an envelope, from which they vanish. The deck is spread, and one of the cards is seen face up in the middle. Below it, face down, is the second card: two complete and normal cards again. Weston's routine establishes both face-to-face and back-to-back fusions of two cards, and the open use of a double-backed card and a double-faced card. Despite the establishment of these central ideas for the fusion plot, his routine went entirely overlooked in the later progression of fusion card effects.
Several years later, signatures made their way into the fold. The first fusion of signed cards, one signed by a spectator on its face, the second by the performer on its back, is Art Spring's “Matched Cards” in The Pallbearers Review, Vol. 4, No. 10, Aug. 1969, p. 278.
Four issues later, Karl Fulves added a note regarding Spring's effect (see The Pallbearers Review, Vol. 5, No. 2, Dec. 1969, p. 308). Fulves first mentioned Houghton's trick. He then described an effect he recalled he thought originated in England in which two chosen cards, unsigned, are returned to the deck, one face down, the other face up. When the deck is spread, the face-up selection is found fused, back to back with the face-down selection, creating a double-faced card. This trick has yet to be identified.
Despite these developments, fusion effects with cards didn't begin to become really popular with magicians until the publication of “Fusion” by Gene Maze, Richard Kaufman and David Arthur in Kaufman's CardWorks, 1981, p. 47. Strictly speaking, this isn’t a fusion routine, regardless of the title. The cards weren’t presented as fused but merely “stuck together” (which, while similar, is conceptually different). A double-backer was used, which was then apparently split back into two cards, giving the merger no permanency.
1981 also saw the publication of Paul Harris and Looy Simonoff's “The Beast with Two Backs” in Close-Up Fantasies Finalé, 1981, p. 113. This trick ended with two cards permanently fused together in the form of a red/blue double-backer.
The first published variation to fuse together two spectator-signed selections is “Hotfoot” by Jay Sankey, in Sankey Panky, 1986, p. 76.
The citations above offer points of origin and some highlights in the evolution of the fusion effect. To explore the subject in greater detail, refer to:
Prior to the fusion effect being applied to cards, it was suggested for coins and billiard balls.
Prior to cards being fused face-to-face or back-to-back, George Fairclough published a method of fusing cards end-to-end. His trick, “The Mystical Aces and Kings” appeared in The Sphinx, Vol. 6 No. 8, Oct. 1907, p. 93.