I would like for example to share the location sensors of my mobile devices with other HA installations to be able to provide the location of my phone to relatives but also receive the location of phones that are connected to different HA installations. It would be nice to synchronize those specific entities.
A primary and secondary definition would be awesome that would enable to control even more than two installations eg. for an apartment with multiple flats. Some central devices could be shared towards all flats e.g. camera on the apartment front door which would reside in the primary installation. On the other hand each flat has their individual devices which stay local to each secondary installation. Some local secondary sensor data might be interesting for optimizing the whole building.
Some of this seems to be possible with this custom integration: GitHub - custom-components/remote_homeassistant: Links multiple home-assistant instances together