UPDATE: It also looks like Apple is adding support for AirPlay 2 in this beta as tvOS 11.4 references the feature.
However, this feature (as it was before) requires the HomePod software to be on the latest beta, which is currently unavailable. Attempting to create a stereo pair will prompt that “a HomePod software update is required to complete stereo pair setup.”
This is nothing new as users got the same alert when attempting to create a stereo pair in early iOS 11.3 betas. Hopefully, if Apple plans to launch this feature alongside iOS 11.4, the company will roll out a HomePod beta to developers.
iOS 11.4 is set to debut in June, which gives Apple at least two months to test and possibly remove the feature entirely.