How do I combine two Apple Accounts for a seamless privacy settings update?

I have two Apple IDs:

a Mac.com ID which I have used for years and make purchases on (apps, shows, subscriptions &c.) and a me.com ID which I use primarily for email &c.

Whenever I update my iOS I'm asked to update my privacy settings - using my Mac.com address: however - as I go through the 2FA process, Apple won't let me use this address.

At the Apple Store they have suggested I ask Apple Online Support to 'combine' my IDs - so I simply use my me.com address and all purchases and subscriptions remain.

Any assistance on this would be appreciated

[Re-Titled by Moderator]