It says in the app you can choose two factor ID for each line on the account but only one of my lines is listed as an option, I need to get codes on both my phones.
Only lines that are registered for MyVerizon access will show up in that "Two-factor authentication" section to turn on/off. 2FA is sent via a push notification through the MyVerizon app installed on that device or a text message link/code to accept or deny the access request. Note any device changes can disallow the 2FA as a option when logging into MyVerizon until the security timeframe elapses for it to appear.
If you are referring to 2FA for logging into My Verizon, I suspect the reason why only one line will let you do it is because both lines are under one account, and only the Account Owner's line is the one which is able to login to My Verizon.
