Every instance is doing this, right now. When you post on one instance, every instance with a single subscriber to the community gets sent a copy.
On kbin, even the media is stored on the instance. It helps distribute the load. Instances share posts between instances which can then each support many users.
In terms of "taking over" a community. Not so easy.
See, I could take [email protected] from my instance, do some SQL hacking and turn it into a local community. But, that would only work on my instance. Everyone else would still be following the original and the original would still exist.
For it to work it needs to be a co-ordinated community move.
Mods pick an instance with as much of the original data already federated as possible. They communicate the new home. People start subscribing, the old group is made read only with a message linking the new one.
To keep existing posts though other instances would also need to SQL hack. So adding some features to communicate and automate the SQL effort would be a nice thing.