I was reading Villains United and there are two Lex Luthors. Whereabouts (years, story arc, event) did these two LL get introduced first?
Crisis on Infinite Earths is the first appearance of the “other” Lex.
Check out Crisis on Infinite Earths like BobcatZoidberg said for the background of the character, and then look into Infinite Crisis for how it’s explained and resolved. The other three miniseries leading into Infinite Crisis were The OMAC Project, Day of Vengeance, and Rann/Thanagar War, if you want to be totally up to date on the background, but all you really need to know is Crisis on Infinite Earths (which is excellent, if a little hammy).