iOS 11.3 leaves AirPlay 2 out
AirPlay 2 appears prominently on Apple's official iOS page, but the feature has been removed from iOS 11.3 beta 3 and has not made it in the final software either.
With AirPlay 2, you will be able to wirelessly stream audio from an iPhone or iPad to multiple devices simultaneously, so you could have two HomePods blasting out sound together for a fuller, more refined sound experience, or you can have a HomePod and an Apple TV connected simultaneously for multi-room audio.
AirPlay 2 would allow pairing two HomePods together, but it will come later, likely with iOS 11.4
Interestingly, AirPlay 2 was available to try in iOS 11.3 beta 1 and beta 2, but apparently Apple engineers could not finalize the feature in time and it will launch with a future iOS update, likely iOS 11.4.
source: Apple Support