This fall, they face a separation of a different kind – college.
The Houston sisters were born conjoined, sharing a liver, chest and bile ducts. The surgery to separate them was considered essential, but risky.
"Importantly, the heart was not shared," Dr. Kevin Lally, Professor and Chairman of the Department of Pediatric Surgery at the Children's Memorial Hermann Hospital told ABC News. "So I didn’t think that there was going to be anything that would prevent us from doing a separation."
Both sisters not only survived, they thrived. In May, they were co-valedictorians at Lutheran High North. They stood at opposite ends of the stage to give their speech.
"We started the speech each saying one word and went back and forth to make it funny," Caitlin tells PEOPLE.
Courtesy Copeland Family
"We don’t think about our past on a daily basis," Emily tells PEOPLE. "But when we are reminded what we went through it makes turning 18 that much more special."
The twins do everything together – from their homework, assistant teaching at Sunday school at their local church and serving on the student council. Their lockers at school were even right next to each other.
'It Will Be Tough'This fall, though, Emily will go to the University of Houston to major in hotel management and Caitlin to Concordia University Texas in Austin to study education.
"It's the first time we're going to be separated permanently," Caitlin told ABC. "And so … kind of freaking out a little bit."
So is her sister.
"It's scary," Emily told ABC. "If I start overthinking it, I get really, really sad."
It's tough for their parents, John and Crystal, both 44, too.
"I will go from having a full schedule bringing them to all of their extracurricular activities to having nothing on my schedule," Crystal, an office manager, tells PEOPLE. "I'm going to go from moving 1,000 miles an hour to a complete dead step."
The twins have a plan, though.
"We bought iPads so we will FaceTime every day and still do homework together," says Caitlin. "It will be tough being apart but we will definitely visit each other."