"Two Women, Two Fantasies" by DG (dionysian1@hotmail.com). The author's stories are archived at http://www.io.com/~thebear/dgidx.htm.
Two couples are partying together and decided that it would be great to have the wives write down their deepest, darkest fantasies and share them with their husbands, who will help bring the fantasies to fruition. A few days later one couple has a bdsm fantasy, the other is getting the massage of her life.
I can't tell you much more, except to say that the author adds a postscript indicating that he laughed his ass off when he thought of the idea. The story also reminded me of the time my sister-in-law was in charge of assigning names for the Christmas present exchange, and she assigned everyone her own name.
Actually, the title is wrong: there are MORE than two fantasies in this story.
I started this story with some apprehension, but I wound up laughing my own ass off.