Multi-factor authentication is now supported by the Second Life viewer in addition to login at https://secondlife.com
Please see this article for full explanation of the changes. Note that it does not yet reflect Viewer support.
https://community.secondlife.com/blogs/entry/8952-introducing-multi-factor-authentication-for-your-second-life-account/#:~:text=Go%20to%20accounts.secondlife.com,regularly%20and%20choose%20strong%20passwords.