Nothing is impossible, but in order for two chicks to hatch from one egg the egg has to:
1) be so big as to hold twice the volume as a standard egg. Usually the strongest embryo will thwart the smaller one which will expire well before full term and and it's remains will poison the egg content, also killing the larger embryo.
2) On the rare occasion that both chicks survive to the hatching stage in an extra large egg, they will both have to pip within a very short time of each other, which is most unlikely. The chick that is at the pointed end of the egg would also have to be strong enough to break through the inner membrane and the outer shell at the same time because at that stage it starts to breathe with its lungs, and there is no air-cell at the pointed end. The normal routine is that when a chick is ready to hatch the blood vessels attached to the membrane which has nourished it with nutrients and oxygen close off while the remaining yolk is drawn in. The gradial lack of oxygen makes the chick convulse which causes the egg-tooth (a small pointed bit on the beak) to break through into the air-cell, then having a rest and pumping blood into it's temporarily enlarged neck muscle before it resumes the strenious task of breaking through the shell.