I’ve had a similar issue with android tv and Apple TV connected to it. I wanted one tv in HomeKit that can control both of them through one HomeKit remote turn on and off both of them and as well have apps from both of them in one source list.
But now to get to a possible solution.
First install media_player_template integration through hacs and configure it so it can control your chromecast ( following the template modify to what you’d like to accomplish with it). Then using universal media player integration create a new media player whose child entity is the media player template one and android tv one ( or if you’ve configured the app list in the media player template leave out android tv from children list). Turn on and off goes for both entities and volume up and down can be either of them or both of them. Once you’ve configured the universal media player, add the device_class: tv at the end. In home assistant HomeKit add this universal media player as an accessory. Once that is done, you should be able to control them both. Just remember, to get them both to work together in HomeKit you should also get the states for on and off and make an automation that can turn on and off either depending on which one is on or off.
Just pointing you to a direction I took, might help