Google to introduce multi-account sign-in:
Users of Google services will be able to sign-in to different accounts in the same browser soon, as the company is rolling out multi-account sign-in to its users, reports Josh Lowensohn of cnet.

According to Google Operating System blog, the company is in the process of introducing a new feature that lets users cycle through up to three of their registered Google accounts without having to re-identify their credentials. It means that the user is allowed to sign in to more than one account using the same browser session. However, the user can only sign in to a maximum of three accounts at a time. Adding to it, users will be able to switch from one to another with a simple drop down menu.
The new system requires opting in from a user's Google account management page. Once enabled, the user can switch accounts in Gmail, Calendar, Reader, Code, and Google's Sites products through the use of a drop-down menu at the top right-hand corner of compatible Google sites.

However, this solution does not truly enable users to be logged in to two different accounts at once. Users cannot run two tabs of Gmail with a different account in the same browser instance.