Separating linked iPhones , creating another Apple ID for second iPhone , keeping new sim in second phone and keeping older active sim in other phone without loosing data
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iPhone 8, iOS 14
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Your data is associated with exactly one Apple ID.
Therefore, for the iPhone acquiring the new Apple ID, the first thing to do is:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Then populate that iPhone only with items belonging to the new ID.
If you wish: