It would take a plulgin to duplicate the topic and its revisions. But then what if one or the other topics keeps making revisions?
I think the thing to do is
- start the topic in the secret place
- discuss it, edit it,
- when it’s ready to be made public, you wrench, select posts, select the OP/first post and move it to a new topic
The moved topic moves with its edits. The original one stays in the original topic, with its edits. The new one is in the new topic with the edits.
I think this does what you want.