Yes, they would be, because you are still logged into your Apple account. You wouldn't use iPhone Handoff for border crossing unless you wanted to leave one phone at home and transfer your number to the iPhone you are taking with you when you leave the country. Still, the phones would have identical data on them unless you turned off syncing for certain apps, such as iMessage (messages).
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Wouldn't all the apps, app data, your photos, messages etc be synced between your phones?
Yes, they would be, because you are still logged into your Apple account. You wouldn't use iPhone Handoff for border crossing unless you wanted to leave one phone at home and transfer your number to the iPhone you are taking with you when you leave the country. Still, the phones would have identical data on them unless you turned off syncing for certain apps, such as iMessage (messages).