Congrats to JediMSieer who guessed what happened...
Yes, Nat and Lyn were siamese sisters as you can see in this old "photo"
When their parents died, they suddendly realized they were alone, they had no friends, no boyfriend, and the rest of the family was not very excited to have to take care of a "freak" who can't wear normal clothes, drive a car etc...
So the two girls decided to be separated in order to be more independant and to try to live a life as normal as possible. even if not completely normal, girls with missing limbs do not scary people as much as girl with two heads, three arms and three legs.
Nat Always got control of the third arm (which was a right one), while Lyn always controlled the third leg (which was the a left one) so, the limb sharing was obvious when it was time for operation.
I hope they are both happy now they are separated..