This is a picture of the setup for the simultaneous fall experiment. The arrow indicates the trigger which, when pushed up, will release both balls at the same time.
This experiment demonstrates that the force of gravity acts on all objects the same, no matter their mass, size, or path during falling. Also, this experiment shows that vertical forces and horizontal forces act completely independently of each other. The sound of the balls hitting is very clearly one sound even though two balls are falling.
This is an illustration of the experiment.
Optional: metal pans(or other) to enhance single sound of hitting bottom