A new feature is coming for only for the HomePod that will add an immersive home theater experience when paired with an Apple TV 4K. To get the 5.1, 7.1 surround, and Dolby Atmos, you need to pair one or two HomePod speakers to the Apple TV. This feature requires the spatial sound support of HomePod, so it isn’t available for the HomePod mini.
You can’t have a HomePod and HomePod mini connected to Apple TV simultaneously, but you can have two HomePod minis connected to Apple TV to create stereo sound. That’s a great option if you’re not interested in the new home theater experience.