To maintain a state of zero momentum after they start pulling the larger student will move at a smaller velocity and, therefore, not travel as far. The smaller student will move at a much higher velocity and, consequently, roll for a greater distance. It is important that the wheels of the wheel chair be aligned correctly before the pulling starts and that some one be there to catch the students before they crash in to each other. There are a lot of variations that one can do with this. Two students of similar size can pull.
Students can push off of each others hands instead of pull.
The results will probally not be good enough to make good quantitative measurements but they will be quite sufficient for making a qualitative estimate of what should take place.