Like Abby and Brittany, conjoined twins have distinct personalities. Abby is the more gregarious and outspoken of the two, according to their large group of friends. But Brittany is more "laid back and chill" and has a "weird" sense of humor.
Abby wins some arguments and Brittany wins others. Their friends laugh when they overhear the twins "in the background bickering" over the choice of a blouse or a pair of shoes.
Moir has seen the same with his patients.
"These kids show a fusion of spirit, where you see their separate personalities, even though they are conjoined," he says. "Their own individual uniqueness is a joy to watch."
The psychological bonds between conjoined twins are well-known.
"It's a special bond," says Moir. "They share a body throughout their development in utero and it continues after they are delivered. They share a bloodstream. You give one a shot and the other is immunized, one catches a cold and so does the other. How intimately and intricately they are designed."
As for the Hensel twins, when Abby drinks caffeinated coffee, Brittany can feel it immediately and her heart, which is weaker, races.
As for their emotional bond, one friend remarks, "They finish each other's sentences -- they both know what the other is thinking and what she is going to say. It makes me smile all the time."
When conjoined twins are not separated, there are "strong medical reasons not to do so," according to Moir. Usually, they share organ systems that cannot survive independently. Surgery is still rare and only half survive.
Families with twins require much support.
"We have been truly in awe of the parents who have brought their twins to us," he says. "They will do whatever they can for their kids and they love them no matter what."
Abby and Brittany say they are "nervous" and know there are challenges ahead, such as finding work and launching out on their own.
"There is so much uncertainty about what is about to happen," says Abby.
But their mother, Patty Hensel, says she has confidence in the twins' approach to life: "It doesn't matter what the challenges are, they go after it."