Merge Two Apple ID's

You can't merge Apple 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. https://support.apple.com/en-us/HT201060

Family Sharing - Set up. https://support.apple.com/en-us/HT201088

I have tried the family sharing solution but I am still missing content on different devices. This does not seem to be a resolution.

Is there a way to transfer all purchases, photos etc to just one account?

I’ve to Apple IDs and when I try to add the other email, I get this message “invalid email “ but actually that email is correct and it’s my second Apple ID. Even though the two IDs look alike it’s not the same example

silktag@hotmail.com & silktag@gmail.com I don’t know why I get this invalid message

You can't transfer purchases between Apple IDs. Purchases are tied to the ID that made the purchase.

Well that doesn't seem to be quite customer centric.

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You can't add the email address from one account to another account. They are separate Apple IDs