If Db_A already has XenForo running, export it. Then import it into an empty database, like Db_B, and copy the files you're using for that instance to another directory or account where you want to use a second xenforo license. Then point the config.php of this copy to Database_B
You have now mirrored your xenforo instance.
You run two instances, and therefor will need two licenses.
You can login and delete threads and users and forums on Db_B, and they won't be deleted from Db_A.
I hope that explains the logic behind multiple instances, mirrored instances, ..