How do I merge 2 Apple id’s
How do I merge my Apple ID on my iPhone with my Apple ID on my iPad?
iPad Pro, iPadOS 13
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You can't merge Apple IDs. Pick the ID with the most data/purchases and on the device that is using the other ID, change the 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. If possible, also set up Family Sharing between the IDs 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.