How to undo list merge

Unfortunately, there is no direct way to undo merge.

If both lists were rule-based, you can recreate them and they will populate with the contacts that meet the 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 the merge.

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

To delete unnecessary contacts from a newly recreated list;

•  Create a rules-based list with a rule 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 the static list.