In fact, the idea of Lex Luthor and the Joker teaming up is the subject of the classic animated movie The Batman/Superman Movie: World's Finest, which marks the first crossover between Superman: The Animated Series and Batman: The Animated Series all the way back in 1997.

As Comicbook.com detailed, Luthor and the Joker teamed up in a three-part 1997 episode of Batman: The Animated Series and Superman: The Animated Series. The Joker proposes a deal to Luthor, offering to take down Superman for a substantial price. Ultimately, Superman and Batman team up and stop the dynamic duo of villains.