How to undo list merge

Unfortunately there is no direct way to undo merge

If both lists were rule based You could recreate them and this will populate them with the contacts that meet criteria of the rules. This solution will not add contacts added to those lists manually or by automation

If both lists were static there is no way to undo merge

If you merged static list into rule based or opposite you can recreate rule based list and delete unnecessary contacts from the static list

To delete unnecessary contacts from newly recreated list you can create a rules-based list that says a member of the merged list but not a member of the new rules-based list you created. This will allow you to get all the members who were in static list