Is it possible to un-merge tickets?
No, it is not possible to un-merge tickets. Ticket merges are final and cannot be undone (see Merging tickets). If two tickets have been merged by mistake, one option would be creating follow up ticket from the closed ticket that was merged by mistake and removing any unwanted CC's and ticket fields pulled over from the merge. You can read more about this here, Creating a follow-up for a closed ticket.