Hi Paul! The answers to your questions are predicated on knowing where those contacts were originally stored. They were not stored locally (on the iPhones themselves), else the behavior would not have occurred whereby the deletions were mirrored between devices -- that much we know! So, they were originally stored either in iCloud or on another Mail/Contacts/Calendars account configured on both of your iPhones.
Let's start by assuming these contacts were stored in iCloud. If that's indeed the case, you may well be able to restore them using the following directions:
- Visit iCloud.com in a web browser of choice, such as Safari or Chrome.
- Enter your Apple ID and password.
- If you have two-factor authentication turned on, you'll need a trusted device to generate a code to log into iCloud and access the settings.
- Click on the "Settings" app in the main iCloud menu.
- Scroll down to the bottom of the page until you get to the "Advanced" section.
- Click on the content you need to restore. Options are files stored in iCloud Drive, a list of contacts, or your Calendar and Reminders.
Apple regularly archives a list of Contacts, which can be restored to an iOS device at any time. Restoring a list of contacts from the archive will replace the contacts on all iOS devices connected to your iCloud account, while archiving the current list of contacts to ensure nothing is lost. Contacts can't be restored one by one -- this is an all or nothing batch operation.
Once you've restored your contacts, you can "separate" or disassociate your iPhones by ensuring they are not both signed into the same iCloud account. To do this, choose either iPhone and:
- Tap Settings > iCloud on your iOS device.
- Scroll down and tap Sign Out, then tap Sign Out to confirm. If you're using iOS 7 or earlier, tap Delete Account, then tap Delete to confirm.
If it turns out that your contacts were originally stored in another account (for instance, you're syncing contacts from Google or Outlook and the deleted contact originally came from there), please let me know where they came from and we'll investigate whether other recovery options may be available.