You can log in as both the backup account (from the Backup Accounts view) or as the company account (from the Companies view) if you’re using your partner credentials:

But if you’re already logged in as the company account (from the Companies view), then you’ll be unable to log in as the backup account. It’s impossible to use “login as” option recursively.

In this case, you need to click the link Resume session as in the upper part, and then the option to log in as the backup account will be available:

In other words you can use “login as” option only once. To use it again, you have to resume the session first.