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.
Here are the instructions, per Splatoon's Tumblr:
On the controller selection screen for Battle Dojo, press and hold down on the +Control Pad, press and hold B, and then press ZR and ZL.
When using a Wii U Pro Controller, first press ZL and ZR to select that as your controller, and then press the 2 Button on the Wii Remote Plus to activate it.
This reminds me of the time people made paper-plate steering-wheels for their Wii Remotes to avoid buying the Wii Wheel to play Mario Kart 6. Anyway, if you can get it to work, congratulations! Now you have full motion control support for both players in local multiplayer.