Time constraints and technical limitations nearly left the ability to play the entire game with a friend on the cutting room floor.
http://www.nintendoworldreport.com/news/31213
New Super Mario Bros. 2's cooperative mode, which allows two players to progress through the entire main game together, was nearly unable to be included in the final product, the latest Iwata Asks revealed.
During the development of New Super Mario Bros. 2, Takashi Tezuka, who was overseeing the development of the game, suggested that a robust cooperative mode be added. Rather than the competitive mode that was featured in the original New Super Mario Bros., Tezuka instead envisioned a mode more akin to that in New Super Mario Bros. Wii.
While this inclusion seems like a no-brainer, it was met with hesitation from Yusuke Amano and Masaaki Ishikawa, the Director and Art Director of New Super Mario Bros. 2, respectively. Though a prototype of the mode was up and running, the pair didn't believe that they would have enough time to go back and retrofit the levels that had already been completed to accommodate a possible second player. More than that, they also pointed out that, unlike other entries in the series, New Super Mario Bros. 2's level structure allowed for a lot of exploration, not just simple left-to-right progression. This would prove troublesome in terms of how to focus the camera on both players.
The solution? Have the camera focus on one player, while allowing the two to compete for who that player is. Amano and Ishikawa rationalized it in a few ways, including how a less-skilled player can improve by watching a better one, and how two players can work together to collect the multitude of coins present on the screen at one time.
So, after some trepidation, the cooperative mode was included in the final product after all.