Reminder -- the iTunes ability to play to multiple speakers is not AirPlay 2, but is prett...
Many years ago, Apple added the ability to play a music stream to multiple AirPlay-compatible devices, but this is not AirPlay 2 and doesn't work the quite the same way.

Yes, you can stream to multiple speakers with it. Yes, the speakers are more or less in sync with each other, allowing you to fill a whole house with music.
However, the iTunes streaming method to hit multiple targets doesn't pay any attention to any latency between the different general classes of speakers: wired, Bluetooth, and AirPlay. If you've got a Bluetooth speaker connected to your Mac, and you play to that plus a speaker wired to the computer, there can be a noticeable difference in the playback timeline because of buffering -- with a set of AirPlay speakers somewhere in the middle of the two.
If you're aware of this, though, you can make smart choices about it. For instance, playing only to similarly connected speakers, like just to AirPlay speakers, or only to locally wired speakers to the computer in question should keep the latency differences down to a bare minimum.
There is an app called WHAALE Multiroom Player for iOS that allows for similar behavior from an iPhone. It's not a free solution, but it allows users to set up a "soundstage" of multiple devices, in much the same way that iTunes allows now -- and AirPlay 2 will when it finally arrives.

Yes, you can stream to multiple speakers with it. Yes, the speakers are more or less in sync with each other, allowing you to fill a whole house with music.
However, the iTunes streaming method to hit multiple targets doesn't pay any attention to any latency between the different general classes of speakers: wired, Bluetooth, and AirPlay. If you've got a Bluetooth speaker connected to your Mac, and you play to that plus a speaker wired to the computer, there can be a noticeable difference in the playback timeline because of buffering -- with a set of AirPlay speakers somewhere in the middle of the two.
If you're aware of this, though, you can make smart choices about it. For instance, playing only to similarly connected speakers, like just to AirPlay speakers, or only to locally wired speakers to the computer in question should keep the latency differences down to a bare minimum.
There is an app called WHAALE Multiroom Player for iOS that allows for similar behavior from an iPhone. It's not a free solution, but it allows users to set up a "soundstage" of multiple devices, in much the same way that iTunes allows now -- and AirPlay 2 will when it finally arrives.
Comments
-Direct audio attached to a Mac
-Airplay from the Mac to 3 Apple TVs (1 forth generation over HDMI, 2 second generation over analog out) attached to stereo receivers.
-Airplay from the Mac to 2 iHome WS2s over wifi
-Airplay from the Mac to 4 iHome WS3 over wifi
-Airplay from the Mac to Raspberry PI attached to stereo receiver.
all the above at the same time and all are in sync via iTunes on a Mac.
If I want to my iPhone provide the audio source, I can use a combination Airfoil Satellite for Mac (and stream to the Mac) and Airfoil for Mac (which distributes the incoming audio to other speakers) and Airfoil remote for iOS ( to control individual volumes of those outputs).
I don't use the iPhone to drive the audio source so much as a remote for iTunes on the Mac because I don't like draining my iPhone's battery.
If I do end up buying a HomePod, I'm not sure I would utilize Airplay 2 much for that same reason. The battery life.
An an even better use case would be the HomePod turned around and pushed the music it is fetching from Apple Music itself to airplay 1 devices.
“The room correction applied after probing its own position isn’t simplistic DSP of frequency response, as the speaker has seven drivers that are used to create a beamforming speaker array,. so they can direct specific sound in specific directions. The only other speakers that do this is the Beolab 90, and Lexicon SL-1. The Beolab 90 is $85,000/pair”
http://www.loopinsight.com/2018/01/24/on-homepod-and-audio-quality/
Here we go again. HomePod supports voice control for Apple Music, iTunes Match, and iTunes purchase history. It works as an AirPlay endpoint for any other audio source exactly as any other airplay wifi speaker.
I have a $300 AirPlay speaker that I’ll be replacing with a HP. Mine doesn’t do anything except sit there as a dumb speaker until you beam an audio source to it, and it doesn’t even sound good. The HP clearly adds way more value than anything else in the entire AirPlay speaker category, but because its new voice controls don’t work with Spotify it’s all of a sudden a major calamity. Yeah no.
The Home Max sounds terrible and you get a google microphone in your home which is just creepy.
Show me a smart speaker pair for $700 that beats the HomePod because there isn't one.
You can use regular stereo speakers, amp etc for a $700 pair but even those don't have voice control let alone 360 sound.
Some of use are familiar with what is out there you know.