Apple ID

If you have the same ID, if you change your ID, you will change her ID. You need to create a new ID for yourself.

Apple ID - create a new.      

Apple ID – Create         

Family Sharing.         

Family Sharing - Set up.        

When changing to the new ID, 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. Also set up Family Sharing to keep access to your apps/purchases. 

How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.        

What to do after you change your Apple ID or password - Apple Support.     

Any AppleID only has a single primary login email assigned. It is also intended to be used by one person as their personal and private login ID for all Apple services.

So one of you should keep the one current AppleID and the other needs to make their own. You can use Family Sharing to share all existing purchases made on the current AppleID.