To login to multiple gmail accounts simultaneously in the same browser, do the following-
1. Go to www.google.com/accounts and login using your gmail credentials.
2. Click ‘Edit’ in ‘Multiple sign-in’ options.
3. Select ‘On – Use multiple Google accounts in the same web browser’ and check all 4 options. Then click ‘Save’ button.
4. Sign out of Google accounts when the above settings have been saved.
5. Now login to your gmail account (make sure you are in standard mode and not the basic HTML mode) and in the drop-down menu near your gmail account name, select ‘Sign in to another account…’
6. Enter the credentials for your other gmail account.
7. Now you can click the drop-down menu near your gmail account name and select the account you want to switch to.