how to separate two devices from same apple id

Here is the whole procedure: 

• If you don’t have a recent backup, create one on your computer or in iCloud (just for security, you probably won’t need it)→How to back up your iPhone, iPad, and iPod touch - Apple Support
• Go to Settings/[apple ID name] and tap Sign Out
• If you sync to iCloud you will be asked if you want to keep the content that syncs; say yes
• Tap to create a new Apple ID
• If you sync to iCloud, go to Settings/Apple ID name]/iCloud and turn on the switches for items you want to sync.
• Log in to the same email accounts you had on the shared Apple ID if they have logged out (they probably did not, unless one of them was an iCloud email address)

Contacts, Calendars, Notes, and Reminders sync to email accounts, so if you are logged in you will still have all of them. You will also still have all photos.

Next, decide who will be the family “organizer” and have that person set up Family Sharing→Set up Family Sharing - Apple Support

You can share all previous and future purchases; here is more information on that→Share with your family - Apple Support

(Courtesy of Lawrence Finch)