You have two choices. To view both mailboxes at once, go to tools, options, mail settings tab, click on email accounts, choose to view or change and click next, go into more options, then the advanced tab and add the account. It will now show on your folder list under the other account.
However if you have to send an email as the other account you need to set up a second profile.Close outlook and go into the control panel. Change the view to classic or small icons depending on your Operating System. Open the mail icon and then click show profiles. Select to “prompt for profile.” and click OK. Open outlook and when the prompt window comes up choose “new” and set up the other account as its own profile.