Access two Gmail accounts at once in the same browser - Official Gmail Blog

This is Amazing ? Who do not have two or more Gmail accounts? So it is great that you can combine two Gimal Accounts together.

summery
Now, you can visit google.com/accounts and click the link next to "Multiple sign-in." After you sign into your first account, you can sign in with up to two additional accounts from the new accounts menu in the upper right hand corner of Gmail, then easily toggle back and forth between them. You can even open multiple Gmail tabs — one for each of your accounts.

Old School method.
With Google Chrome you can use the incognito windows and normal window to access two Gmail Accounts at the same time.

Access two Gmail accounts at once in the same browser - Official Gmail Blog