Source: https://www.engadget.com/2018/11/19/chromecast-google-home-speaker-groups/
The last paragraph has some interesting info:
Per XDA Developers , all generations of Chromecast devices should be compatible with the feature. Smart displays including Google's own Home Hub and the LG Xboom WK9 will be able to be added to speaker groups in the coming weeks, according to VentureBeat .
I liken the smart displays to Google Home devices, so not quite Android TV devices. But we shall see if this feature also comes to the platform.
So does this mean I will be able to play audio on my androidTV and speaker at the same time or do I need a Chromecast specifically?