You can merge two calendar folders that keep the calendars separate but view them as one using the calendar overlay feature.

In Outlook 2010 you can overlay two or more calendars

  • Right-click on the name of the calendar folder and choose Overlay from the drop-down menu. 

 

Another option is to click the left-hand arrow, as shown for Diane's calendar, which will overlay the calendar over the other existing calendars.  Clicking the right arrow allows the calendars to become separate again.