Dear P.O.V.A. MANAGEMENT,
Welcome to Microsoft Community.
According to your description, you merged two email accounts, and you want to undo it. You can try my suggestions below to solve this problem, I hope my suggestions can help you.
First of all, I would like to confirm with you whether the merging of email accounts you mentioned means adding an alias to your email account? If you add a third-party domain alias to your email account, you can remove the alias without affecting the account. If you have added an email alias for a Microsoft domain (@Outlook.com, @Hotmail.com, @live.com, @msn.com) to your email account and you want to remove the alias, you're permanently deleting the alias and it can't be associated with any Microsoft account again.
You can refer to related instructions: Add or remove an email alias in Outlook.com - Microsoft Support
In addition, you mentioned that the merged email address did not receive your family storage invitation, I want to explain to you that emails are synchronized with the server of the corresponding email domain name, if your merged primary email account is an email account with Microsoft domain name, You can check if your family's stored invitations are in your inbox by logging into the Outlook server (Outlook.live.com)
If your merged primary email account is an email account with a third-party domain, you can log into the Web side of the corresponding email domain to check if your family stored invitations are in your inbox.
Finally, regarding the transfer of all cloud files from your two mailboxes to your home storage in your main email (third email address), you can re-publish a thread under the Office section, and the relevant engineers responsible for Office products will be better able to help you solve this problem.
I hope this information was helpful. If you have any questions, please let me know, I will be happy to help you further.
Have a good day.
Best regards
Jim - MSFT | Microsoft Community Support Specialist