How to link one Windows Xbox account to 2 Microsoft accounts

Thanks a lot for all the answers!

Your "Xbox" account is, for all intents and purposes, your Microsoft Account, so you can't tie a 'gamertag' to two seperate Microsoft Accounts.

What you CAN do, which might not fit what you're hoping, is change the email address of your Microsoft Account,
or add the other 'gamertag' as an alias to one account with the steps here.

I don't think that's really what you're looking for, but it's the closest option available that I am aware of.

I want the accounts linked to be able to use the same XBox account with my new Microsoft account

You could
add the second Microsoft Account as a sign in alias to that one account.

However, depending on what you want to achieve with the "joining" of those two Microsoft Accounts it might not be the solution you're looking for.

Maybe if you explained the reason for adding two Microsoft Accounts to one Windows Xbox Account, we might be of more help.

No problem! Best of luck and let us know if you have any questions or problems in any of the process you try for resolution!