Yeah that is a pain but I agree with @rlithgow1 - separate accounts are the most straightforward way of handling it...
You could use the community app "Hub Connect" which is kind of like the Hub Mesh feature but a bit more flexible as it allows for sharing devices across various networks/internet etc. You could set up a VPN link to your cottage and then share it's the devices with your main house and then use one google instance to control all of it.