An unassisted successful hatching - as you said - has not been recorded (yet). The hatching process is com[licated and involves the chick chipping against the inside which turns it around in minute proportions. For two chicks to do this in syncro would be extremely rare. The dominant chick usually forces its weight against the weaker one which often dies in the egg. If this happens prior to 2 days before hatching due, the dead chick will poison the egg content and both will die.
All chicks that hatch from a egg clutch are twins, triplet, quads, quins etc. depending on the number of eggs. They are no different from each other than the two in one egg. To get an genetical twin there needs to be a normal single yolked egg where the embryo (zygote) splits at an very early stage of development... this inevitably results in one or both of the chicks dying because of room and available yolk for food.