GMail has a mail fetcher on their system, meaning that you can manage more than one email account from your single GMail account. You can POP the email messages on your other account and read it on your primary account.
What happened if someone sent you a message on your secondary account and it was downloaded (using POP3) into your main account? Would you be able to reply as the original recipient (using your secondary account's domain) ? The answer is YES. When you add the account, there's an option to specify whether you want to be able to reply the message using the same recipient or use a single identity for all account.
Recently, i joined SlackBasics translation project which are now integrated in Google Code. The current administrator of SlackBasics (Marco) has given me a new email account @slackbasics.org. It uses Google Apps to host the email account using GMail infrastructure.
I already have GMail account, so why would i have to logged in to two accounts when i can manage all of them in one single account? This is why i add my @slackbasics account into my primary GMail account and from there, i can read and send any email that goes through my @slackbasics.org account without having to logged in to the account
For those who are curious on how to do that, check the GMail settings and look on Accounts tab. Look on Get mail from other accounts. You can add more than one account (i don't know the limitations on the accounts) and after you setup the account, you can modify the Send mail as part to change how will you reply/send new messages between those two accounts. You can change the settings at compose/reply window though, so i guess it's not too strict that you have to define it first.