The tower is high enough so that both of the balls have reached their terminal velocity
When an object falls it accelerates downwards due to the the force of gravity (
The friction acting on the ball increases as its velocity increases until it reaches a point where the force due to friction is equal to the force due to gravity.
The forces acting on the ball are now balanced so the object remains moving at a constant velocity due to Newton's First Law of Motion
This constant velocity is an object's terminal velocity and it is the highest speed that object can fall due to gravity.
Both balls are travelling at the same velocity when they impact the ground so they must have both reached their terminal velocity, meaning that the tower must be high enough for the downwards-thrown ball to accelerate to terminal velocity before impact. If this is so then the upwards-thrown ball will also reach its terminal velocity since it has further to fall.