Hi sari85 , you shouldn't need to login again or do 2FA when changing between files. If you are, it may be to do with the way you are switching.
For Desktop
When switching between files, it is best to go to the top menu and go to file > open. Then you can navigate between files without having to do 2FA or re-login
For Browser
When switching between files, click on the business name on the top right of the screen, then click 'switch business'. Also will not require 2FA or login again.
If/when you do experience an inactivity logout however, you will need to re-enter your email and password. But you will not required to complete 2FA.