How do I unlink two iPhones on the same account

Is your other iPhone lost or stolen? Or is the problem that you never set up your own Apple IDs?

If the phone is lost or stolen, and Find My was turned on, you would want to mark the phone as Lost, and maybe even to attempt to remotely erase it. But you’d want to keep it linked to your device list so it would be a “brick” for thieves.

If you are sharing an Apple ID with another family member, then. You ought to disentangle that by giving each person their own Apple ID.