Apple ID

Create a new Apple ID for one phone. There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. Also set up Family Sharing to keep access to your apps/purchases. Also see Winston Churchill's response in the below discussion

Apple ID – Create

How to transfer all files from one iCloud account to another

 Apple ID email address or password - What to do after you change

Family Sharing

Family Sharing - Set up

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

Family Sharing and location

Creating separate Apple ID's will truly separate the two phones but if you want both phones to share the same Apple ID try this:

Goto: Settings(Gearbox icon)>Messages>Send & Receive> then select or unselect the phone numbers and or email addresses you want that phone to send and receive messages from.

You will probably need to do the same thing with FaceTime.