You can do it in the same window, even. Like I said, it’s just a bit complicated. But since I can’t seem to find a guide out there, I’ll detail how I did it. It’s in four parts.
Part 1: Enable checking your mail in one account.
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[li]Pick one account as your main account. For me it’s the one that uses my real name.[/li][li]In that account, go to Mail Settings > Accounts and Import.[/li][li]Under “Check mail using POP3,” click the “Add POP3 email account” button. [/li][li]Type in the other email account there, and click Next. Be sure to include the @gmail.com part.[/li][li]Type in your password on the next page, and then choose the bottom two options.[/li][li]Click “Add Account.”[/li][/ol]
Part 2: Enable sending mail from either account.
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[li]You should still be on the Account and Import screen from part 1. If not, go there again.[/li][li]Under “Send Mail as,” and “When replying to a message,” choose “Reply from the same address the message was sent to.”[/li][li]Also under “Send Mail as,” click the “Send Mail from another address” button.[/li][li]Put in the name you use on the second account, as well as the full email address. Again, don’t forget the @gmail.com part.[/li][li]On the next page, choose “Send Verification.” It can take a bit to arrive, so go on to part three.[/li][/ol]
Part 3: Make things more convenient.
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[li]Go back to the Tab that has your settings, and choose Labs.[/li][li]Using the box at the top, search for and enable “Multiple Inboxes” and “Refresh POP accounts.” Don’t forget to click “Save Changes” when you are finished.[/li][li]Go back to your Settings. Choose the new option “Multiple Inboxes.”[/li][li]Under Pane0, type “in:” (without the quotes) followed by your second email address. If you customized the label in part 1, use that instead. On the textbox to the right, choose a label for the account. I use the account name.[/li][li]Decide whether you want your other account on top, bottom, or right side of your main inbox, and choose the appropriate option[/li][li]Delete the contents of any other textboxes on the page, and click the Save Changes button.[/li][/ol]
Part 4: Finish Part 2.
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[li]Go back to your Inbox.[/li][li]If your message from Gmail hasn’t arrived yet, click the new Refresh button that looks like two arrows chasing each other.[/li][li]Open the message from Gmail.[/li][li]Follow the instructions inside. (Click on the long URL.)[/li][/ol]
It sounds worse than it is. And it’s more convenient once you start adding even more accounts (they don’t even have to be from Gmail) or have to check your email elsewhere.