If you don't want to create a new Apple ID, what you can do on the phones will be limited. You will only be able to use Messages/FaceTime on one phone because if both phone are signed into Messages/FaceTime, you will see all messages visible on both phones. Any data synced to iCloud will show on both phones. If you want 2 independent phones, you will need to create another Apple ID.
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.
Apple ID - create a new
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