Apple ID must be different than notification email. Why?

Are you sure it requires a notification email address? The article kind of implies you do not. Two-factor does not use email for anything critical. It won't send a verification code by email.

Rescue email address

"If you have two-factor authentication or two-step verification, you have a notification email address instead of a rescue email address and won't need to remember security questions."

"You can have a rescue email address only if you don't have two-factor authentication or two-step verification. A rescue email is optional but recommended."

Maybe askApple?

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

Look at this article to see the purposes behind the different addresses you can enter. About your Apple ID email addresses - Apple Support