If this post from Splatoon's official Tumblr is legit, you can strap a Wii Remote Plus to a twin-analog Wii controller and give a second live player motion-controlled aiming in the game's Battle Dojo mode.
This is key because otherwise, only the Wii U GamePad — and only one is supported on the Wii U — has the motion-control aiming features. But with the hack Nintendo describes, a Classic Controller, Classic Controller Pro, or Wii U Pro Controller can be straddled to a Wii Remote Plus to achieve the same effect.