I have witnessed it myself, watching approaching aircraft, pretty close to the perimeter, while another is still taking off. Have always thought "these guys obviously know what they're doing". Not so sure now.
vested interest - You can sleep safe tonight and every other night. Viewed from the side it would be hard to distinguish between departures from the RIGHT and LEFT runways as well as arrivals on either, unless you are in between! In which case they are a long way apart, as in Charles De Gaul etc.
In any major airport using parallel runways, the picture can be confusing to the most seasoned professional not involved in ATC at the time. ATC are a highly professional crowd who have no intention of risking your life and their job. If you have any faith in statistics take a look, accidents in which ATC played a significant part very few, pilots planning to be involved in an accident, somewhere between 0 and 0+, a very little.
Enjoy your flights, it is the safest form of transport on Earth!