This article explains that a user can register and use multiple verification methods for multi-factor authentication (MFA), such as a combination of Salesforce Authenticator and a third-party authenticator app.
To register a second or subsequent verification method, you must connect it from Advanced User Details within your personal settings after logging in. The detailed registration procedure depends on the verification method chosen by the user.
Note:
When you [Connect] ([Register]) an additional verification method, a verification code is not sent to your email address like it is for the first verification method. Instead, you will be asked to verify your identity using a verification method that is already connected. For information on how to select a verification method when logging in after connecting a second or subsequent method, refer to this article.
Common multi-factor authentication (MFA) troubleshooting
Salesforce Multi-Factor Authentication FAQ
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