Can't be done directly in Outlook unless you create a custom folder homepage. However, you can easily accomplish essentially the same thing by:
1) Open Outlook to the Calendar folder (or the Inbox);
2) Right-click on the Inbox (or the Calendar) in either the Outlook Bar or the Folder List, and select "Open in New Window";
3) Right-click on the Windows Taskbar, and select "Tile Windows Horizontally" or "Tile Windows Vertically".
You'll have the two folders displayed on your screen either side-by-side or one-on-top-of-the-other.