Is it possible to un-merge tickets?
No, it is not possible to un-merge tickets. The ticket that is merged moves to a Closed status which cannot be reversed and the ticket is no longer editable. For more information, see the article: Merging tickets.
If two tickets were merged by mistake, create a follow-up ticket from the closed ticket that was merged by mistake. Then remove any unwanted CC's and ticket fields pulled over from the merge.