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.