Tween class if moved mc passed point start another tween

Hey Kirupa Flash Gurus

Now I am using the Tween Class to move some boxes in my movie, now I can move the first mc and then another mc after the first tween has finished with onMotionFinished but I was hoping someone could help with how do I start the second tween when the first mc has passed a certain _x coordinate.

ie starting the second mc moving while the first tween is still moving.

Regs,
Jacko