To activate Auto-Advance, go to Labs tab under Settings. The user has the option to go to the next email, go to the previous email or return to the list of emails in a stream. Click Enable, then click Save.
If you own or manage multiple Gmail accounts, you will basically have to exit your current account to log in to the other account, or use multiple browsers at once to access them. However, Gmail's new feature "multiple account sign-in" will help solve this.
Logging in to multiple accounts at the same time allows you to log in to 2 accounts, and switch back and forth between them (or open each card for an account and log in simultaneously). Note that multiple accounts with the same login only work on one browser.
To enable this feature, go to google.com/accounts and select Edit then " Multiple sign-in" . You will have to check some of the dialog boxes to show that you understand how to use this feature. When completed, access the account page and click Edit next to the email address to add another account.