This article from the blog at DCComics.com identifies the story in Wonder Woman #100 (AUG58) as the first multiverse story:  "The Forest of Giants!/The Challenge of Dimension X" (16 pages) featuring an identical twin of Wonder Woman. Just after that, 10-year-old me picked up two consecutive issues of Adventure Comics off the spinner rack. Adventure Comics #252(SEP58) and #253(Oct58) had the first…