that is abnormal for parent birds to do that unless they notice that one of the chick is sick but that is very rare. Other than that, one possibility is that one of the chick may be a lot stronger and greedier so it hogs the feed all the time and the weaker one cant get enough feed. This happens commonly.
Fadedracer is correct. During feeding time, take out the chick that hogs the feed and let the other one get its fill then put put it back.