tvOS Angry Birds Go Multiplayer

I've been completely unable to get split screen multiplayer to work with Angry Birds Go. It keeps asking to attach a second controller. The Apple TV Remote app on iOS (in addition to the Siri remote) is communicating just fine with the Apple TV. Rovio has no documentation (or useful support) other than the YouTube video of it working in split screen.

Has anyone got this to work with the iPhone as a second controller?

Thanks in advance,