How do I separate two iPhones on same Apple ID?

There are many things that you are not able to separate when using the same ID on two devices. The Apple ID is supposed to be for an individual person so they can easily start work on one device and pick up where they left off on another. Each family member should have their own Apple ID.

Make sure that each family member has a unique Apple ID - Apple Support

You might see your Call History, Messages, Voicemails being shared and all services that use iCloud.

If you want to log out of your Apple ID on your device, head over to Settings > [your name] > Scroll down and click Sign Out. Be sure there are no restrictions that are preventing you from signing out.

For more information: Sign out of iCloud on your devices - Apple Support

In addition to Mac Jim ID's excellent explanation, you might want to look at this user tip:

How to “unshare” an Apple ID - Apple Community