How to access my second iCloud account?

Since you do not say why you cannot simply sign into it I don't see how we can help you until you do. If you want help, you need to tell us how you are attempting to sign in and what makes you think it isn't working. We need the exact wording of any error messages.

Note that iPhones can only sign into one account at a time for most services. If you want to get access to a second account, log in with a web browser.