From the looks of the trailer, this could be a hint at co-op modes or a two player campaign even. Nightwing, Robin, and Catwoman all make their return and the dual takedowns show promise of the cooperation required for some of the bigger enemies.
From an official press release to PC Gamer :
"For the first time ever, watch Batman: Arkham Knight’s exciting new 'Dual Play' feature in action, which will allow players to seamlessly switch between The Dark Knight and his allies including Robin, Nightwing, and Catwoman in FreeFlow Combat."
Batman: Arkham Knight will be released June 23rd.
 - PC Gamer - Batman: Arkham Knight Trailer Shows Familiar Faces - http://www.pcgamer.com/batman-arkham-knight-trailer-shows-familiar-faces/