Good news for users of multiple Google accounts (i.e. – one for work and one for personal): now you can sign-in to more than one of them from the same browser window.
Before you can turn it on, Google wants you to be aware of a few caveats, most notably that not all products support multiple sign-in and offline mode for Gmail and Google Calendar won’t work.
Looks like this feature has only been released for some accounts or only in some parts of the world. When I head to Google Accounts I am not provided with any option like this and am shown this when I try to access the multiple sign-in settings page via Getting started: Enabling multiple sign-in:
The page you requested is invalid.