There are two methods to overlay your calendar, we will be demonstrating them one after the other.
1. The first method to overlay your calendars is by making sure they are visible with the checkbox next to the calendar, please reference the image below to see the calendar's checkbox. Once they are checked and your calendars are visible, you can click on the Overlay arrow on the Calendar tab to overlay them.
2. You will then see your calendars overlayed like below. If you choose to separate them from overlay, you can click on the arrow indicated below.
The second method is by,
1. Right clicking to tab of the second calendar, and selecting the Overlay option.
2. Then to remove the Overlay, simply right click the tab of the calendar again, and select Overlay.
These are the two methods available to Overlay.