For the three people who purchased more than one HomePod, I guess this is a big deal? I dunno, this doesn't do anything for me.
“Just days before iOS 12 is unveiled to the public, Apple is fulfilling two promises made a year ago in iOS 11. First announced earlier this morning, iOS 11.4 is now available for iPhone and iPad. The new update includes AirPlay 2, HomePod stereo pairing, Messages in iCloud, and more...
...iOS 11.4 is available for iPhone, iPad, and iPod touch models capable of running iOS 11. The update introduces AirPlay 2, the next-gen version of Apple’s media streaming technology, which reduces latency and supports multi-room playback from iOS for the first time.”