How can a merged forum be unmerged?

There is no automatic way to undo this, you only options would be restoring the database to a point before the merge (you would lose all changes after that occurred after the backup) or manually recreating the forum and moving the threads one by one.

There is no automatic way to undo this, you only options would be restoring the database to a point before the merge (you would lose all changes after that occurred after the backup) or manually recreating the forum and moving the threads one by one.