Encounter: Laundry Fight (Chapter 2)
A good place for this is during the fist fight in the prison laundry room (end of chapter 2). You can go there via encounter select.
When one of your friends punches an enemy you must attack that same enemy. Then you will see an animation where both of you knock out the enemy together. There are infinite enemies spawning in that room, so you can take your time (just don't attack the boss enemy or the encounter will end).