When you have two Calendars or when accessing your Calendar and another Users Shared Calendar by default you will be given Side by Side view.
Example: Your Mailbox and the Test Calendar that we created above are both being displayed in Side by Side mode.
Figure 1: Two calendars in side by side mode
In order to change this view and have these Calendars displayed in Overlaid Mode click the Small Arrow on the tab of the secondary Calendar, in this example that would be the Test Calendar:
Figure 2: Use arrow for overlay mode
Once you have clicked this Arrow your Calendars will now be displayed in Overlaid Mode:
You will now be able to see both Calendar Appointments displayed on the same Calendar, obviously very useful for scheduling purposes. Notice the color coding is intact based on which Calendar each appointment is from.
Figure 3: Overlay mode view
In order to exit Overlaid mode simply click the same Small Arrow. Your view will now be returned to Side by Side mode.
Figure 4: Use arrow to exit overlay mode