Log into either of your new Gmail account. Click the Settings link and select Forwarding POP/IMAP on the Settings toolbar. Select Forward a Copy of Incoming Mail to in the Forwarding section. Enter your first Gmail address into the Add a Forwarding Email Address and click Next. When you see a message informing you that a confirmation code has been sent to verify permission, click OK.
Sign out of this email account and into your primary Gmail account. Open the confirmation email message and click the link to confirm that you do wish to allow email messages from your second account to be forwarded to the this one.
Click the Settings link in the upper right corner. Click Labs on the Settings toolbar to view the available Labs, which are experimental new features. Scroll down the list of labs until you see Multiple Inboxes. The labs are in alphabetical order, so it should be relatively easy to find.
Click the Enable button to begin using the Multiple Inboxes feature. Scroll back up the page and click the Save Changes button. This takes you back to your Gmail Inbox. Click Settings again and you will notice that you now have a Multiple Inboxes option on the Settings toolbar.
Go to Multiple Inboxes and type to:Second@gmail.com, replacing the word Second with your second Gmail account login name. Select an option under Extra Panels Positioning and click the Save Changes button.
Sign out of your primary Gmail account and back into the second account. Click the Settings link and select Forwarding POP/IMAP on the Settings toolbar. Select Forward a Copy of Incoming Mail to in the Forwarding section again and make sure the primary email address is still listed. Click the Save Changes button.
When you log into your primary account once more, you will be able to view, create, reply to and forward email messages for both accounts in the same window. You may wish to use labels to manage your separate Gmail accounts.