Unmerge 2 Accounts? Help

We need help separating two accounts that were merged about 4 years ago. These both were originally owned by me and 2 years ago I sold 1 of them. The new owners need these 2 accounts to be separated. From what we understand this is a Login issue that Amazon needs to do. We have tried many different ways to unmerge the login for these 2 accounts. This is something that is now a very large concern for each of us as we (and employees) both now have legal, and financial access, information, and modification abilities. This needs to be corrected.

How is it that Amazon in all its Tech can’t unmerge the logins for 2 merged accounts?

Please Help!