So most services providers don't have a means to merge your accounts although I do have a few paid subscriber-ships where the companies can, but they are the rarity in the crowds.
The only way I know of merging your accounts is to literally move the content from one account to another account (i.e. Google accounts with play, drive and mail or Microsoft Outlook).
I find it very disappointing that the Microsoft accounts, like Apple, are generally a paid account yet they do not have the means to merge client accounts as part of the paid-for services. Interestingly enough Amazon does allow and will actually help with
merging your account information, transactions and digital media.
I realize this is a Microsoft forum and the thread is about merging your accounts which I am still in favor of getting, but the only way I know of doing this is to manually do this.
You'll need to open up both accounts, in separate transaction windows and move your free-stuff first, that's the easiest part.
Then selectively identify the paid content you have for that account and update the account owner information (principally the email and credit card data) so you can keep the licensed ownership of the content.
With something like your email, the best way to do that is to store 100% of your email content on a local drive (I have 4 archives going back to 1996, why I don't really know). Once you have the local copies of the emails and attachments you can disconnect
from that account and close that account manually.
Unfortunately NONE of this is provided guidance from Microsoft and I can't for the life of the planet understand why.
For everything else you'll need to just "let it go".
So you know we're not alone with this problem for general Microsoft accounts, I had my original XBOX account setup under one email address and then changed it to another email address, that was like a 4-ton lift, and I still lost 6 movies I'd paid $20 for
back in the day, and that was frustrating.