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Note: You can unlock this after completing Level 12 - The One With The T. rex Mech.

Batman: Unlocks during the main story in Level 12 - The One With The T. rex Mech.
The Joker: Unlocks during the main story in Level 1 - New Kid On The Block.

While in the hub world or free play switch one of your characters holding [ Register or Signin to view external links. ] to Batman and the other character to The Joker. With Joker stand next to Batman in the background. Press [ Register or Signin to view external links. ] for selfie mode make sure Batman is in the background you might have to turn around with the camara. Once you have the perfect shot press [ Register or Signin to view external links. ] and the achievement will unlock.