Place your device on a flat surface and play with a friend. Assign each player to one side. Juggle with multiple balls get them bouncing at an even height. One player must kick the balls less aggressively until both balls are in synch. When this happens, it is easier to manage both balls.
Various strategies
Depending on when the screen is tapped, the ball will react differently. With timing, you can control the ball's height. You can start tapping the ball as soon as it falls just above your character's head. Tapping before this point will result in a miss. If you wait a bit longer to tap and hit the ball, it will rise higher, giving you more time to get ready for the next tap. Tap the screen just as the ball drops below your head. If done correctly, the arc the ball follows on its way to the other player will be higher.