I have two different Apple IDs for my iPad and my iPhone. How do I fix it?

You ask two questions:

How you deal with two IDs depends upon if you you have important things both accounts. If you do not, simply sign out of the less important account and sign in on that device with the more important one. Ideally erase the device in between times. If you have critical things on both, decide which one has more apps and other purchases and you'll keep that one and merge things from the other that you can. Others may tell you this is not possible but you can go a long way to doing it. Let me know and I can post more details.

Here's how you change the email address (Apple ID) you use for your Apple Account while retaining everything in your account:

1. Go to https://appleid.apple.com and sign in.

2. In the Sign-In and Security section, choose Apple ID.

3. Enter the email address that you want to use as your Apple ID.

4. Choose Change Apple ID.

5. If you changed your Apple ID to a third-party email address, check your old ID email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support

6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support

For more details, read the document: "Change your Apple ID" --> Change your Apple ID - Apple Support

Apple says: "When you make certain updates to your Apple ID, we’ll send you an email to help you complete the update. For example, you might have reset your password, reset your security questions, or added a new email address to your account." If Apple tries sending you a verification email and you do not receive it, read this support article --> If you didn't receive your verification or reset email - Apple Support

Read about what to do after [or before] you change your Apple ID email address or password - What to do after you change your Apple ID or password - Apple Support

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (go to this document: How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.

If you still need help this page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support

If you have a mobile number, see the document: "Use your mobile phone number as your Apple ID" --> Use your mobile phone number as your Apple ID - Apple Support

From:

Change your Apple ID - Apple Support

- There's special instructions if you want to change your Apple ID to an @icloud.com address.

"If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later."