You can't have two numbers on it as the text can only be sent to the one you have supplied. If you want to change it you need to turn it off which you can do by entering the recovery code as you can then supply a different mobile number if you want.
To turn off 2FA sign in with the recovery code so at the login select "can't sign in with the code" link. You'll then be asked for the recovery code.
When you signed up to 2FA you were given a recovery code so if you didn't make a note of it then check the email you were also sent.
Help can also be found here on how to turn this off and/or change your number:
https://www.sky.com/help/home/sky-yahoo-mail/two-step-verif/articles/two-step-verification
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