Read speed between two ultrasonic sensors
There are several issues with these ultrasonic modules and your code:


How fast are these objects moving? If you don't have any reflection, it'll take almost 20ms for one measure. So precision will be low.  
What if the object is moving in oposite direction?
What if the object stucked between?


and so on.

Anyway Laser interrupter circuit (or similar) would be much more suitable for this job.

Or if you want to use ultrasonic, you can use Doppler effect. But it's not possible with those PING))) modules (or clones).