This tutorial shows you how to view two (or more) calendars side by side and overlay them in Outlook 2007 (and Outlook 2010/2013).
Outlook 2003 supports side-by-side calendars but does not support overlaying the calendars. Older versions of Outlook do not support opening multiple calendars like this; you'll need to right click on the calendar and choose "Open in New Window" then arrange the windows on your screen so you can see all of them at the same time.
To switch between overlay and side-by-side mode:
Click on the arrow on the tab
- or -
Right-click on the folder name and choose View in [other] Mode.