Apple released iOS 11.4 last week, which included AirPlay 2, the new version of Apple’s music and video streaming framework. And they yesterday released macOS 10.13.5, but AirPlay 2 is not part of this update. As you can see below, in the AirPlay menu in iTunes, the two HomePods in my bedroom show as two devices.
But I have connected these two HomePods as a stereo pair, and you can see this in the AirPlay menu on the iPhone:
It’s surprising that Apple hasn’t rolled AirPlay 2 into macOS. You have long been able to stream to multiple speakers from the Mac, using iTunes, and that is new on iOS with AirPlay 2, but the inability for the Mac to be able to use AirPlay 2 and see a stereo pair of HomePods is surprising.