I have two Apple IDs. My wife and I use one Apple ID for our two iPhones. I want my wife to use the second Apple ID for her phone (we never wanted to share phone and messages, but now do). When I try to change her Apple ID get email is not available.

If you sign her out of the shared ID and then sign in with the 2nd ID, will that work? Don't attempt to change the ID she is currently using. Sign out/in.

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. See Winston Churchill's response in the below discussion.

Apple ID – Create

Apple ID - create a new

How to transfer all files from one iCloud account to another

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

You can try setting up Family Sharing between the 2 IDs if you want to be able to share purchases made with the IDs.

Family Sharing

Family Sharing - Set up

You are welcome.

Life was good for many years until my wife and I noticed that messages, contacts and calls started to be shared. I spent a week studying how to move her onto another Apple ID. Signing out of Apple and iCloud accounts first helped the project. Thanks for your many years of help.