In polyphonic parts it is sometimes useful to write two sticks to 1 note head. One stick up and the other down. The key combination <alt-E> takes care of that. In other cases it is more useful to place both notes right next to each other. If both notes get an explicit stem direction, the note heads will overlap and two stems will appear on one head. If only one of them has an explicit stem direction they appear next to each other.