“Unmerging” Amazon Accounts? Any tips or Tricks?

Have any of you had success with “unmerging” accounts? I owned 2 Amazon accounts and 4 years ago I merged them on Seller Central to make it easier to log in and switch between the two. A year ago I sold one of the companies and we are still trying to get amazon to separate them because I can still access the old company and they can access mine. Been trying for over 8 months and 7 cases with no success. Any tips or ideas?