Another point of intersection between J. Eric Holmes & L. Sprague de Camp is Dragon magazine. In the 1970s, Gary Gygax was corresponding with a number of authors, including at least Holmes, de Camp, Fritz Leiber and Gardner Fox, which led to each having one or more short stories published in Dragon (Holmes' first book, Mahars of Pellucidar, was published in 1976). This lead to the coincidental simultaneous appearance or mention of de Camp's and Holmes' work in several issues of Dragon.
Dragons #15 and #16 (June and July 1978) have a two-part reprint of The Green Magician, the final Harold Shea/Incomplete Enchanter story by de Camp and Fletcher Pratt, which was out-of-print at the time. The cover of Dragon #15 (pictured above), by Dave Trampier, illustrates a scene from this story. Interspersed with the story in each issue is a full-page advertisement for Gen Con XI (Aug 17-20, 1978) that mentions "special D&D Events for beginners DMed by J. Eric Holmes" (pg 30).
In addition, Dragon #16 also contains a letter to the editor from Holmes, a rebuttal to a reader's letter in Dragon #14 criticizing the Cthulhu mythos article in Dragon #12 by Holmes and Kuntz.
Dragon #31 (Nov 1979) has Holmes' first Boinger and Zereth short story for Dragon, Trollshead. In the Sorceror's Scroll column in the same issue, Gary Gygax mentions the Incomplete Enchanter as inspiring the G-series of modules (pg 28-29).
Dragon #46 (Feb 1981) has Holmes' second Boinger and Zereth short story, The Sorcerer's Jewel. This issue also has a Boot Hill scenario by Roger E. Moore called "This Here's Tyrannosaurus Tex (pg 28-29), which cites a book by de Camp and his wife, The Day of the Dinosaur (1968), for inspiration.