Justice League: Crisis on Two Earths.
There are multiverses in the DCU – Earth 1, 2, S etc. Well on one such Earth there is the Crime Syndicate. Run by bad guy versions of the JLA. In that world Lex Luthor is a good guy with Joker (called the jester). Well this movie explores what happens when the two worlds meet.
Really good movie that gets serious half way through when – I wont tell you, go watch it!