Having two HomePods means the already-impressive bass performance is even stronger, and you can crank up the volume even more. Just like with a single unit, you can get pretty close to max volume without the music distorting. That was valuable for basically all types of music I tried, from the lush orchestration in Howard Shore's Lord of the Rings soundtracks to Julien Baker's breathtaking and minimal performance on her album Turn Out the Lights. Blasting some electro-pop like Cardiknox and Broods brought out the speakers' excellent bass, and more aggressive albums like the two recent Nine Inch Nails EPs and some old-school Metallica brought the complex layers of the songs to the forefront along with chest-pounding beats.
It's hard to tell exactly how the HomePod's "spatial awareness" affects music when playing it in stereo, but it sounded nearly as good from across the room as it did sitting directly in front of them. The stereo separation isn't as noticeable when not sitting dead center, but they still sounds excellent. It's a well-balanced listening experience, similar to using a single HomePod, but with welcome increases in overall volume and bass performance.The speakers' 360-degree design makes the music feel like it's equally present regardless of where you are in the room, rather than it coming direct from a single point.
Dual HomePods are still not as loud or as good as a pair of the fantastic Sonos Play:5 speakers, and Sonos doesn't tie you to one music service. But, the Play:5 don't offer voice control, has a more complicated setup and audio tuning process, and cost $500 each ($150 more than a HomePod). Sonos does currently have a deal where you can get two Play:5s for $900, but that's still $200 more than a pair of HomePods.
Two HomePods can also potentially upgrade your TV's sound -- if, of course, you're using an Apple TV. The latest tvOS update added an option to output audio from the set-top box to the HomePod, either a single speaker or a stereo pair. It's not going to be an upgrade if you've got a proper surround sound system, but if you're still making do with your TV's built-in speaker (like me) it's definitely worth trying. Still, I wouldn't advise anyone buy them specifically for your TV. Your $700 would probably be better spent on dedicated home theater gear, and the HomePods only work with things you can play on Apple TV. Also, volume output when watching Apple TV was strangely quiet -- I typically turned the HomePod nearly all the way up while watching movies.
There are some additional oddities in how the Apple TV and HomePod work together. For starters, asking Siri to play some music would turn on the Apple TV, because the two devices are linked together. That's annoying, but the alternative is de-selecting HomePod as an output option when you're done watching TV, which means you'll need to re-select it when you want to watch something else. Either way, you're jumping in and out of settings a lot, and it's clear evidence that Apple didn't really intend anyone to use this as a main home theater speaker.
As for multi-room audio, controlling that is pretty simple, as well. If you start a song on one HomePod and want to play it in another room, you can go to the iOS Control Center and long-press on the media controls to see all the AirPlay 2-capable devices hooked up to your home network. From there, press on the speaker playing music and select which additional devices you want to play music through. Google and Sonos work similarly; in their apps, you can see all the devices you have available and create groups, or leave them set up as individual speakers that can play different songs simultaneously. As for voice commands, you can just ask Siri to play a song, album or playlist and include the speaker(s) that you want it to play on.