Use Incognito Mode to Log in to Multiple Accounts Simultaneously
Recently Google launched a feature that lets you Access two Gmail accounts at once in the same browser. This is nice but I prefer to keep doing it the same way I've done it till now: Opening an Incognito window.
Incognito Mode
In Google Chrome, to get to Incognito mode press CTRL+SHIFT+N in the keyboard, or click the wrench icon on the browser toolbar and select New incognito window.
In Incognito mode, pages won't appear in your browser history or search history, and they won't leave other traces, like cookies, on your computer after you close the incognito window. Any files you download or bookmarks you create will be preserved, however" (quote from the opening page of incognito mode).

Since no session data is stored or transfered between the regular window and incognito window, you can stay loged in to all your accounts, and in addition log in to other accounts in the incognito window.
Better then Google's 'Multiple sign-in'
There are tow main advantages to using an Incognito window to access two accounts:
- It's less error prone. I figured it when I tryed to activate this feature (in https://www.google.com/accounts/MultipleSessions), and was asked to sign these conditions:
- The account you’re currently using is displayed at the top of most Google product pages, or at the bottom of most pages on mobile devices. Always check to make sure you’re using the account you want.
- Not all Google products support multiple sign-in. When switching between Google products, the account you’re currently using may change.
- Google products not listed above will default to the first account that you signed in to using your current web browser session.
- You can access other accounts, not Google's. For example, you can access another Facebook account or any other services.
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- Prevent Major Third Parties From Tracking You With Chrome Extension ‘Disconnect’

Thanks for this tip !