With the previous generations of Apple TV you can use an iOS device with the Apple Remote App. But the question is if Apple will update the app so it will work with the new Apple TV. And in this case, would it be possible to use this as 2nd control in a game?
To implement a two player game, you can require the user to buy a 2nd remote. But as a developer, it is not yet possible to get hold of another remote to start developing. And I'm not sure it is a wise thing to require players to have to buy more touchpad remotes.
The Multipeer Connectivity framework is not supported by tvOS. Game Center peer-to-peer technology is not supported by tvOS.
The last option is to develop your own remote-app for iOS devices, that will connect to your tvOS app through CoreBluetooth or Bonjour network. This require some work, and the App Store will be filled with different remote-apps for different tvOS games.