Two iPhones - different country, same ID?

Any given AppleID can only be signed into one country or region at any one time. The country or region used is not device specific. It is AppleID specific.

If you want to use the App Store/iTunes in each country, you will need a different Apple ID.

No, an email can only be the primary or backup email to one AppleID. It’s simple really - that primary email address IS the AppleID as far as it being a login ID to a specific Apple account. Once you create the AppleID and verify it, it IS the email address used to login with. And the backup contact email must also therefore be specific to that given AppleID.

So one email account = one single Apple login ID (AppleID). And it is the AppleID, so login email address, that defines the account for every single Apple service (iCloud, FaceTime, iMessage, the App, Music and Book stores, etc).

You are welcome.