merge two apple ids

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

See -> Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

Unfortunately you cannot merge Apple ID's. I have been asking for this for ages and finally gave up. I now have two Apple ID's in my family, both for me. I'm not sure what it is exactly that you want to merge. I would expect it to be some purchases, like apps or music. Perhaps creating a family and sharing the purchases that way is a workaround that would work for you?