iOS 11.4 is here, and it brings some long-awaited features to Apple's HomePod speaker, including the ability to create a stereo pair of two HomePods and compatibility with Apple's new multi-room AirPlay 2 standard. Here's how to set everything up: (19659002) Before you get to the fun part, you must first make sure everything has been updated to iOS 11.4. To update your phone, you must go to the Settings → General → Software Update application. As for HomePod, you should automatically find and install the update at midnight, but you can verify that you have updated or force the installation yourself by going to the Start application. There, you can see if a HomePod update is available, download and install the update and manage other features for your HomePod. And in a good option, the startup application can automatically detect if you have several HomePods, and download and install the update for all of them in one shot.
Multiple room audio:
Once you have installed all the updates, the multiple room audio should work immediately. To use it, simply go to your preferred music application on iOS and start playing music through AirPlay, and then select one of your HomePod devices. Now, with iOS 11.4, instead of playing in a single source, the AirPlay 2 compatible speakers will appear in the AirPlay pop-up window with a small checkbox next to them, allowing you to toggle several speakers to play the same song from your iPhone. . This part will work for the HomePod, and presumably any other AirPlay 2 speaker that you will get later on the line.
But with HomePods, there is an extra audio level of several rooms that you can do, where each speaker can have independent audio broadcasts to through Wi-Fi without having to use AirPlay at all. For now, this feature is limited to HomePod, and is limited specifically to Apple Music subscribers (although, hopefully, Apple TV will also get some kind of similar skill later in the line when upgrading to AirPlay 2). To take advantage of that feature, go to the same AirPlay menu as before, which can be accessed from the AirPlay icon in most music applications, or the music widget, and then scroll down below the local track Highest of the iPhone to select one of your HomePod devices. There, you can select an Apple Music track to use as a source and play that track on other AirPlay 2 speakers using the same system of check boxes as when you were playing directly from your iPhone.
Stereo pair:  However, if you plan on keeping your two HomePod together in one room, there is also the option to group them together as a stereo pair.
To create a stereo pair, which treats the two HomePod as a discrete unit for AirPlay and Siri, go back to the start application, select one of your two HomePods, go to the corresponding configuration and press "Create stereo pair …" Then, you will have the option to select your second HomePod to create the pair. Once you have selected it, you will be taken to a second screen to confirm that you have the right and left channels selected correctly: when you tap on each HomePod, it will light up with a sound to make sure it is set correctly, and you can easily change them with another touch, after which you will be ready to continue. When paired, the left HomePod will become the "main" device, handling the microphone duties for Siri requests (which probably will not matter since both speakers will be in the same room in this case)
The grouped HomePod will appear in Apple Music, iTunes, AirPlay and anywhere else as a single pair, which will play the content you transmit in stereo audio. The same AirPlay limitations for multiple rooms will be applied with a stereo-paired HomePod as with just one, where you will still need Apple Music to make the most of the configuration. Lastly, I have not been able to confirm it completely, because I do not have a third HomePod, but it is likely that you can also use the paired stereo audio HomePod with AirPlay 2 multi-room audio, if you wish.
To ungroup the stereo pair and make your two speakers work independently again, go back to the start application, select your paired HomePods and scroll down to press the "Ungroup Accessory" button. You can re-pair them at any time by following the steps above.