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What you could do is have one of the HAs install the custom integration remote-homeassistant. However, even then it would mean that you have to somehow make your instance available outside e.g. VPN, port forwarding, cloud service, ...
This one connects with the instance directly via a long-lived token and allows you to control and read date from the other instance. The good thing though: The configuration.yaml way allows you to specificially include or exclude entities. So with that you could technically only send the entities from slimmelezer+ without having to pass the other info.
I would say the only caveat would be that because the long-lived token is somewhere on the other house's instance, they could technically take it and send commands randomly to your instance :P Maybe with an extra user you could maybe limit the permissions or something similar and create the long-lived token there - haven't tried it though, just a thought.