iOS device as a second controller for AppleTV 4th gen

You won't actually "pair" the phone with the ATV. it connects over wifi if the app is open on the phone. Here is how you do it for Crossy Road

1. Open Crossy Road on your TV.
2. Go to the character picker, then scroll to the far leftfor multiplayer mode. (You'll need at least one character unlocked to open the character picker; if you don't have any unlocked, just play a solo game or two!)
3. On your iOS device, download and open Crossy Road, and tap the arrow in the bottom right corner.
4. Tap the game controller icon.
5. You should be connected after a few moments; you'll see a P2 or P1 in the top right corner to signify the multiplayer connection.
6. Have fun playing with pixels!

Per the iMore website the game has to support the usage of iOS devices as secondary controllers.

http://www.imore.com/what-you-need-know-about-apple-tv-and-game-controllers

iPhone and iPods work as game controllers, too—if your game supports it

You're not just limited to a Siri Remote or third-party controller: The new Apple TV can use your iOS device as an additional controller if the game you're playing supports it. Typically, you'll see iOS controller support for games that have an iPhone or iPad counterpart, like SketchParty.