You can merge Media Lists on Muck Rack in a few simple steps.
Navigate to your Media Lists under Relationships.
Use the checkboxes to select the Media Lists you want to merge.
Click Merge Media Lists in the blue bar.
In the Modal window that appears, you'll be given the option to Create a new list or Merge into a saved list.
The box for Delete original media lists is unchecked by default. Check this box if you want to delete the original lists.
Click Merge media lists.
A confirmation bar will appear indicating the lists are being merged.