Try other combinations for simultaneous firings of the rockets, such as a small amount of water in one and a larger amount of water in the other, or equal amounts of water but one pumped a different number of times. Water rockets can demonstrate Newton's Second Law of Motion where one varies the pressure inside the rocket and the amount of water present. In the first test, neither rocket went very far because the air inside did not have much mass. When water was added to one of the rockets, the effect of mass was demonstrated. Before the air could leave the water rocket, the water had to be expelled first.
Water has a much greater mass than air and it contributed to a much greater thrust.
The rocket with water flew much farther than the rocket filled only with air. By varying the amount of water and air in the rocket and measuring how far the rockets travel, students can see that the thrust of the rocket is dependent on the mass being expelled and the speed of expulsion. Thrust is greatest when mass and acceleration are greatest. Several different versions of water rockets are available. If you can obtain a couple of different models, run comparison flights.