Just as we did in the single source example, we can represent the crests of
each wave with white circles, and the troughs as purple circles. Here is what two
sources look like. Now look at constructive interference across the horizontal middle.
All of the crests from the left source line up exactly with the crests from the right source,
and same for their troughs. So across the middle of the figure, we have constructive
interference.
In the vertical direction, down the middle, we see that the crests from the left source
always cross the crests of the right soruce, and same for their troughs. So down the middle
of the figure, we also have constructive interference.
Now look just off of the vertical center-- just to the left or right are regions where
the crests of the left source overlap the troughs of the right source, and vice versa.
Here, there is destructive interference: the crests of one cancel the troughs of the other.
The next page clarifies this a bit.