2 Apple IDs- how do you merge them?

You don't need to create a new ID because then you would have 3 IDs.

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

Family Sharing.         

Family Sharing - Set up.       

Pick the ID you want to use, then see the last 2 lines of donv's post.

You cannot merge Apple IDs. But, you can create a new Apple ID.

See How to create a new Apple ID https://support.apple.com/en-us/HT204316 See the data implications below.

Some of these things will apply to your situation even though you are not changing your Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

See what Eric Root says on this matter here https://discussions.apple.com/thread/251343949 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747