1 of 2 players is the boss, the other follower (later reverse this)
You start simple and you can expand with various difficulties
if the boss throws the ball with a bounce, the follower throws the ball on breast height to the bass
if the boss throws the ball on breast height, the follower throws the ball with a bounce.
More difficult: if the boss puts down the ball, the follower also puts down the ball, and both walk to the ball of the other, boss throws bounce or breast ball etc
More difficult: if the boss throws the ball in a high arch, then: