Even though it says no "dupes" you CAN still use a guest controller. I unlocked this doing split-screen on my console with a guest controller and a friend online. All three were set on Beginner difficulty to speed the play along. We completed in one sitting without breaks, except as described below in Note #2.
Note #1: as we played through it did NOT update my achievement percentage but still unlocked for me and my online friend when we finished the DLC campaign.
Note #2: as we played through my online friend was twice "kicked" from the game. When this occurred I paused the game, re-invited the online friend, and once he rejoined I restarted from the last/ prior checkpoint to ensure there were no gaps that might have been saved with only two player co-op recorded.