Depending on exactly what you want to do in the guest-house, Hubitat has a novel solution that would meet your needs. You can link two Hubitat hubs together, over Ethernet, to allow physical devices on one hub to show up as virtual devices on the other hub. And, vice versa if desired. You could have both hubs run automations, or configure more of a master/slave relationship. How you use the tools is up to you.
Hubitat also allows you to have SmartThings devices show up in your Hubitat hub. Again, this is using Hubitat developed software. In this configuration, the ST hub sends updates to the Hubitat hub. The communication is one way.
And, of course, there is nothing stopping you from writing you’re own version of these designs to link two SmartThings hubs together.