Announced a year ago Apple approved the HomePod. At that time people already talked about linking two loudspeakers to a stereo pair. IOS 11.4 made the announcement true and added the feature.
Stereo Pair Requirements
To pair two HomePod speakers together, you should have iOS 11.4 or later installed on both devices. Even the iPhone or iPad used for setup should run at least on iOS 11.4.
Setting up two HomePods as stereo speakers
Once you have made sure that all three devices you are using have the latest software version, open the Home app on your iOS device. Both HomePod speakers should be in the same room. If a HomePod is assigned to another room, you can change it now. To do this, hold your finger on the HomePod and tap on "Details". Now choose the right room. Now the option "Create stereo pair" should also appear. Tap on it and then select the second HomePod for the stereo pair. Next, tap the arrow icon between the two speakers to change the pages for output, if necessary. Now tap on "Back" and then on "Done". The stereo pair is already set up.
How to resolve the HomePod stereo pair
If you would like to use your two speakers individually again, then you can release the connection again. To do this, open the Home app on your iOS device and hold your finger on the grouped HomePod. Now select "Details". Now tap Ungroup Devices to resolve the stereo pair.