Viewing calendars side-by-side is nice, but occasionally it is hard to keep track of what events fall on the same day. Luckily, you can choose to overlay calendars to see multiple calendars at once, all in one place.
With multiple calendars open, click the Overlay arrow on a calendar tab.
The second calendar is now stacked on top of the first, and events from both will appear together.
To separate stacked calendars, just click the Overlay arrow again.