Just tried streaming from my phone to Apple TV 4 and Apple TV 4K at the same time using Airplay 2, and the two were not in sync. Obviously, very disappointed. It's probably latency from the chain going through TV and then DAC then amplifier. No ability to adjust signal delay per device, it seems. my Chromecast audios all over the house work flawlessly, including through stand-alone DACs and amplifiers, but the TV is a different animal, I guess. That may be why the ChromeCast itself can't join an audio group. AirFoil for Windows (which is a total mess in any case) was never able to get airplaying quite in sync, either, on the TV setup. Has anyone else had luck?