Thanks to AirPlay 2, they don't need to be used as separate speakers.
Just because you've got a couple of Sonos speakers and a HomePod or two, doesn't mean they have to be used as separate speakers. You can group them in the Home app and play audio from all of them. Here's how.
If you have one of these AirPlay 2 supported Sonos speakers, you can group it with any non-supported Sonos speakers and play through them along with your HomePod.
Update your Sonos speaker for AirPlay 2
Before you can turn your HomePod and Sonos speakers into surround sound, make sure your Sonos speaker is updated with support for AirPlay 2.
If you haven't already done so, you'll need to update your Sonos app to version 9.0. If you're brand new to the Sonos family and have never downloaded the Sonos Controller app before, when you go to download the app, it will already be at version 9.0.
Once you've updated the Sonos Controller app to version 9.0 or higher, you'll find the notification to update your speaker when you launch the app.
Launch the Sonos Controller app on your iPhone.
Tap the More tab. It looks like three dots.
Tap Online Updates.
After the app and your Sonos speaker have been updated, you can add the speaker to your Home app.
Add your Sonos speaker to the Home app
To get your Sonos speaker talking to and working with your HomePod, you'll need to first add it to the Home app on your iPhone.
Launch the Home app on your iPhone.
Tap the Add button in the upper right corner. It looks like a plus symbol.
Tap Add Accessory.
Use the 8 or 9-digit code that came with the startup instructions for your Sonos speaker. If you don't have the code anymore, tap Don't have a Code or Can't Scan? and then tap the Sonos speaker to wake it up. The sync should happen automatically. If it still doesn't connect, follow the steps to troubleshoot adding an accessory.
Once the Sonos speaker is connected, rename the room it's assigned to so it's in the same room as your HomePod.
Turn on Include in Favorites to make sure it shows up on the first page of the Home app whenever you open it.
Now that your Sonos speaker is in the Home app, playing audio through it and your HomePod happens through either the Now Playing widget or by asking Siri.
To play surround sound from your iPhone using the Now Playing widget:
On iPhone X, swipe down from the top right corner of the screen, or on all other iPhone devices, swipe up from the bottom bezel onto the screen to bring up Control Center.
Tap the speaker selection button in the upper right corner of the Now Playing widget.
Tap the AirPlay button.
Tap your Sonos speaker and your HomePod speaker.
Music will now pipe through your house from all the speakers you've selected.
To play surround sound by asking Siri, just make sure to say the room you want audio to play through.
"Hey, Siri. Play the Headbangers playlist in the living room." Siri will pipe the music to all Home app connected speakers that are assigned to that room.
Do you have any questions about playing audio through both your Sonos speakers and your HomePod? Put them in the comments and I'll help you out.