If you’re a GMail user that has multiple GMail accounts, such as an account for work related purposes, as well as an account for personal e-mail, you will be glad to know that you can not access two GMail accounts in the same browser window at the same time.
Previously, in order to access two different Google Accounts at the same time, you would have to log out of one and then into another or you would have to open a separate browser window and then sign in to a different account. Some people may have considered this process to be troublesome, especially if they only needed to take a quick look at something in their other account.
However, it is now possible to log into two different Google Accounts at the same time, in the same browser window, by going to Google.com/accounts and enabling the “Multiple sign-in” feature.
The Multiple sign-in feature not only allows you to access multiple GMail accounts, but also multiple accounts on Google Calendar, Google Code, Google Reader, as well as Google Sites. You can not, however, currently use the “Multiple sign-in” feature to access multiple accounts on services, such as Blogger, as the first account you signed in with will be the account used.
Also, according to the Official GMail Blog, they are still working on making it possible to access multiple accounts in GMail and Google Calendar while in offline mode and suggest that you if you rely on accessing these services in offline mode, you probably should not enable it yet. Also, for now, “Multiple sign-in” is only supported on desktop browsers, so you will not be able to access multiple Google Accounts from a mobile phone at this time.