What I would do is use the normal method of polling the pin for sensor 1, using digitalRead. When it indicated that the sensor went HIGH (or LOW), I'd call pulseIn with the pin for sensor 2, ahd HIGH. When pulseIn returns, it returns either 0 (sensor 2 never went HIGH) or the length of time, in milliseconds, that it waited for the pin to go HIGH. Then, you could calculate the speed of the object of you knew the distance between the sensors.