Google allows users with multiple Gmail account to access all the mail accounts with one single account. This can be done using the Grant access to your account available as part of Google Mail Settings. Click the options icon available on the right hand side of the Google Mail and select Mail settings from the list of available menus.
In the Settings screen, Click the Accounts and Import tab and then navigate to Grant access to your account option.
Specify the other Google Account using which you want to access your account.
Click the Next Step button.
Confirm whether you want to Grant access to your email account. On clicking Send email to grant access button the following confirmation message would appear.
After accepting the confirmation request email received in your master Gmail account, the following Success message would be displayed.
Now when you login to your master Gmail account, click the drop down below the username and select Switch account from the menu list.
The Switch account would display your other Gmail account as shown below. By clicking the mail account you would be to access information from your other account.
This was mainly designed for delegating other users to access your Gmail account but users with multiple users account can also benefit from this feature.