Apparently you want to plot attendance vs date, which wasn't clear.
Set up data as shown, with dates in the first column, then attendance in the first time slot only, attendance in the second time slot only, then attendance in both time slots.
Make a chart of this data. I show a stacked area chart, but I prefer the stacked column chart.
EDIT:
Here's how to do an overlap with Area Charts. Arrange the data as shown, with total for each time slot, and in between the negative of the number of people common to both. Make your area chart. Format 9:00 AM as blue, but with 50% transparency. Format 9 & 11 as no fill. Format 11:00 AM as orange with 50% transparency.
Since the 9 & 11 series is negative, it moves down from the previous (9:00 blue) series, and the next series (11:00 orange) starts from the lower position. Where there is no overlap, the orange just seems to start at the top of the blue. And you can use your own colors that might show the overlap more clearly.
