This effect was invented by Michael Weber. See his “Silverwhere?” in Michael Ammar's The Topit Book, 1983, p. 76. Weber came up with this in 1977 or 1978, after reading Dr. Hiroshi Sawa's spoon-bending effect “Gary Ueller” in Genii, Vol. 41 No. 1, January 1977, p. 25.
As an aside, the method used in Sawa's bending effect—while probably independent invention—owes something to a broken and restored spoon effect by John Gilliland, “Breaking a Spoon”, published in Abracadabra, Vol. 43, No. 1118, July 1, 1967, p. 408.
Michael Weber, working from Sawa's idea, adapted it to transform a spoon into a fork, and also to effect the transposition of those two items. Over the years, variants, embellishments and knock-offs have appeared on the magic market worldwide, most often without mention of Sawa and Weber. Among the embellishments is the the addition of a small piece of magnetic rubber or Blu-tack adhesive-putty to the gaff to make handling easier.
Chad Long has published a different and clever method that uses no extra utensils: “Transputensil” in M-U-M, Vol. 86 No. 10, March 1997, p. 23.