The two HomePod features are expected to be tied to Apple's AirPlay 2 streaming protocol this year, which is also available today with iOS 11.4.
In addition to supporting multiple rooms, AirPlay 2 provides a more robust listening experience. For example, in the original AirPlay format, streaming audio to compatible speakers, including HomePod, often had connection problems.
How to Update HomePod
When you finish updating your iPhone on iOS 11.4, navigate to the Home app and tap Edit and tap  HomePod> Details to access the settings. Then tap Update Software .
If your HomePod automatically reloads and installs firmware, the latest version should already be installed on the device.
After launching the Apple Music App and selecting the AirPlay menu, you will now see two HomePods including location and custom title. On the right side of the pop-up window, users can select the HomePods to which they want audio want to stream.  When you select a single HomePod, music plays in that room, while selecting an additional HomePod enables multi-room audio. When two or more HomePods are selected, they each have their own volume control, which provides granular volume control. A master volume control can also be used to adjust both speakers simultaneously. If you have set your HomePods to be louder than the other one, the volume will be maintained by moving the Master Knob as before.
We can also ask Siri to play the same song on all HomePods or AirPlay 2 supporting speakers at the same time.
In our tests, we heard that the song first started on our white HomePod before the black went up. Audio was perfect between the two speakers.
Siri is smart enough to interact with a user from the next HomePod and can play music on any other HomePod in the house, even if the device is in a different room.
Next, we switched to YouTube and selected a video with audio output to a local HomePod AirPlay. A small pop-up menu asks if you want to replace the playback of this speaker. Tapping on "Replace" will transfer the audio of a HomePod to the YouTube video, while the piece of music from Apple Music will still play on the other device.
First, navigate to the Home app and tap Details on a HomePod. In the Settings menu, tap Create a stereo pair . Select another HomePod, then the room, and assign each Left and Right channel to each HomePod.
If your song supports stereo sound, like "Whatever It Takes" by Imagine Dragons, you can clearly hear the different instruments on each one's homepage. This experience is enhanced when connecting to an Apple TV.
Paired with stereo sound, paired HomePods deliver much more sound than a single HomePod. We tested a single HomePod with our decibel meter and measured a peak of about 85dB at about 6 feet. With the stereo pairing they reached 89db. That's 4 decibels more, that's a bit more than a doubling of the sound.
You not only get louder sound, but also more bass. HomePods provide plenty of bass at low to mid volume levels, but are a bit tight at maximum volume. However, when mated, the bass is much more common at higher listening levels, and the overall experience sounds much better when the volume is at its maximum.