Sorry this feature is not currently available for Chromecast and Google Home or Nest speakers. Please leave feedback for the Google Nest team requesting that this is added.
Thanks for the help, @Cathal_S.
@Gustyfield19, thanks for reaching out. You can group any combination of Google Nest or Home speakers and displays and Chromecast devices together for synchronous music throughout the home. Your music and audio from Chromecast-enabled apps are instantly available to stream. Check the steps below on how to create a speaker group.
Create an audio group
Check this link for more information.
Hope this helps.
Link a device
Make sure your mobile device or tablet is connected to the same Wi-Fi or linked to the same account as your Chromecast, or speaker or display.
Open the Google Home app .
At the top right, tap your account.
Verify that the Google Account shown is the one linked to your Nest speaker or display.
While you can cast using your Nest speakers to your Chromecast, splitting audio and video when casting or streaming to your Chromecast from Netflix isn't supported ― we're always looking for ways to add more features on our devices. You can also send feedback on the Google Home app.
Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.