AFAIK, they held a long range and a short range pila (so 2). They would throw the long range one from a distance, advance a bit, throw the other, then charge. Or if they were being charged, they would simply let their foe do the running and throw one after the other as the enemy approached. These two volleys would falter or break the charge of most foes, thus letting the legionaries charge a relatively stationary line (having disposed of both their pila).