Gmail has added functionality that has been available to Google Apps users for some time - delegating access to an account.
Now, having delegated access to any account to another account, from the last one you can check mail and reply to letters on behalf of the delegated account.
Under the cut screenshots that are completely self-sufficient and without words explain where and what needs to be changed (although I still wrote some explanations).

In the “Settings” select the tab “Accounts”, and see the added option: “Grant Access to your account”. After the confirmation letter, this arrow will appear in another account (just like when multiple accounts are used)

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Now you can work with two accounts, but you are registered in only one. Therefore, letters sent from the delegated account will be signed as (sent from the "delegated account"), just like in the following screenshot:
