Except for closed threads, thread copying is not very useful to the average user, due to how the thread tables were designed (there is no direct mechanism to keep the threads synced since they will have different ID's in the thread table).
We added an additional table to keep track of linked threads, and created handlers for post updating and for the additional caching. Much of this was done in extending the classes in the threadpost datamanager. Anyway, it was a big project and certainly was not worth the time and effort we put into it.
So if you are not working on closed threads, re-think what you are doing.