Gmail recently started merging its App accounts to Gmail. Hence you wont be able to login into both the accounts in the same browser now. Here is a way on which you can still do it. This can be done for people who have multiple gmail accounts and want to be able to switch between each of that in same browser.
First login with the gmail account you want to merge with the primary gmail account(ie login into the secondary gmail account). Go to account settings from the menu on top.
In the Account Settings Edit the multiple sign in option and on it by enabling all the check boxes below it. Now sign out from this account and follow the same procedure in the primary gmail account.
Once done you should get a switch account option in the Mail Menu on the top.
Click on Switch account and select the option sign into another account. This should take you to the gmail login page. Sign into your secondary account. Once you did that the accounts are linked and you should be able to able to select either from the switch account option.
If you need both of them open on the same browser, just right click on the account you want to open in the switch account option and open in a new tab. This allows you to open both of the account in the same browser at a time.