The Crossunder can be done backwards too. Throw two balls simultaneously in two parallel columns, cross your arms & catch each ball in the opposite hand that threw it. Uncross your hands & throw them up again.
The throwing is easy enough, the trick is in the catching. Swing your arms together & lift them upwards, just as you would to throw the balls in a normal crossunder. Your hands should rise up to meet the balls as they fall, bring your arms down at the same speed as the balls, follow the balls & your hands with your eyes, this will give you enough time to adjust your hand positions so each ball falls into place.
This trick is the first step towards Mills Simultaneous 3.