How to add 2nd Apple TV to HomeKit?

1. Verify that each Apple TV box has a unique device name. That is the way to distinguish between the two. Renaming can be done on the Apple TV box, or in the Home app on e.g. iPhone. It will propagate through iCloud.
2. Verify that both have been set up by the same primary user. HomeKit allows for just one administrator. Apple TV boxes registered to other users can’t take part in HomeKit this way.
3. You can set the room for this second Apple TV box to any room of your choosing (even create a new named room), either in the Home app on iPhone, or in the AirPlay and HomeKit settings on the Apple TV box.
4. Within these constraints, additional Apple TV boxes should automatically be part of HomeKit.
5. Each Home can have one active home hub. Other potential hub devices are in standby, by design, ready to take over that ‘job’ when needed.

Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support