You may be using the other one as a notification email address. Even if you removed it, Apple has a bunch of rules about when you can re-use an address with them and when.
Change your Apple Account primary email address
https://support.apple.com/109353
- If you remove an email address that isn’t an iCloud email address from your Apple Account, it can’t be associated with another Apple Account for 30 days for security reasons. After removing an email address from your account, notify anyone who might contact you on Messages, FaceTime, or other Apple apps using that Apple Account email address. After 30 days, another user might be able to associate that email address with an Apple Account for Messages, FaceTime, and more.
If you can’t change your Apple Account primary email address
- If you try to change your Apple Account to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.
- If you see a message that the email address you want to use isn’t available or is already in use, check that you or a family member aren’t already using that email address with a different Apple Account. If a family member is using that email address, remove it from their account, then try to use it.