how do I separate
Create and start using an Apple ID - https://support.apple.com/HT203993
Set up an Apple ID to use in the iTunes Store, App Store, or iBooks Store, and with Apple Music and Apple News - https://support.apple.com/HT204316
Frequently asked questions about Apple ID - http://support.apple.com/kb/HE37
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your/her Apple ID.
Also set up Family Sharing to keep access to your apps. Also see Winston Churchill's response in the below discussion
How to transfer all files from one iCloud account to another
Family Sharing - Use your own Apple ID for Family Sharing
Family Sharing -What types of content can I share?
Family Sharing - If you don‘t see your family‘s shared content
Family Sharing - purchases and payments
Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books
Family Sharing – share iCloud Storage
Family Sharing - Share an iCloud storage plan with your family
how do i seperate the two phones apple id as one is my wifes
