You can log in as both backup account (from the backup accounts view) or company account (from the companies view) if you're using your partner credentials:
But if you are already logged in as company account (from companies view) then you are unable to login as backup account. It is impossible to use "login as" option recursively.
In this case you need to click the link "Resume session as ..." at the top of the page and then option to log in as backup account will be available:
In other words you can use "login as" option only once. In order to use it again you have to "resume session" first.