"They`re so perfect," says their mother, Nicole. "They`re beautiful, and they are so funny and so happy."

At 13 months, Jadon is the rambunctious one. He runs in place and tries to roll off the bed to get away from his brother. When his brother plays with a toy, Jadon rocks back and forth until he can snatch it away.

Anias is the contemplative one who loves listening to his mom read. He`s struggled more in infancy than his brother. He is the silent warrior, his mother says, with a gaze so captivating it`s as if "he looks at your soul."

"I could almost keep them like this," Nicole says.

Their father, Christian, agrees. He says they`ve grown to know both boys` personalities and "we like them the way they are."

And yet, Nicole and Christian have made the decision no parent should ever have to make: Do what they believe is in the best interest of their twin boys, even if it means risking their lives.

"This is so hard," Nicole says, "I`m not going to sugarcoat it."

She says she`s even started "going through a grieving process because I`ve only known them" joined together. She runs her fingers through a swirl of hair the boys share at their foreheads, and says how much she`ll miss doing that.

The couple is sustained by faith. They pray often and seek counsel from their minister. They say they`ve put their trust in God, no matter the outcome. "I don`t know what the answer is to my prayers yet," Nicole says, breaking down crying.

Christian says he recently began to wonder if they`re doing the right thing. Family and friends reassured him -- that they`re making the proper decision.

"I can`t wait to see them as two separate little boys," he says. "That`s what excites me the most. I really just want to know my boys."

Early Thursday morning, Jadon and Anias will be wheeled down the pediatric corridor at Children`s Hospital at Montefiore Medical Center in the Bronx.

Just before the boys enter the sterile area outside the operating room, Nicole and Christian will kiss them goodbye, steeling themselves for up to 20 hours of waiting -- all with the hope two healthy boys will return -- together, but forever separated.

At the invitation of the McDonald family and Montefiore hospital, CNN has been given exclusive access to the remarkable and rare journey of Jadon and Anias. From the heart-wrenching decisions to move forward with surgery to the neurosurgical marvel of the operation itself, to the months of recovery and rehabilitation, CNN will offer a glimpse into a world few have ever seen.

The surgeon leading the operation is Dr. James Goodrich. Considered by many to be the world`s leading neurosurgeon for twins conjoined at the head, he now hopes to become the man who comes between Jadon and Anias and successfully separates them.

"This is about as complicated as it gets," Goodrich says while holding 3D printed models of the boy`s conjoined brains.

Twins joined at the head, called craniopagus twins, are exceedingly rare, occurring in 1 out of every 2.5 million births. About 40% of the twins are stillborn and another one-third die within 24 hours of their birth. Studies have shown 80% of twins joined at the head die of medical complications by the age of 2 if not separated.

Simply making it to this point has been quite a feat for the McDonald boys, who share 1.5 to 2 inches in diameter of brain tissue. While meaningful statistics are hard to gather with something so rare, it is safe to say the separation procedure carries major risks, including the possibility of death or long-term brain damage for one or both boys.

Dr. Goodrich prefers not to dwell on those risks, and instead describes a sort of re-birth he hopes to achieve for Jadon and Anias. "The boys will forever have a second birthday," Goodrich says, to mark the day they were separated.

"They go back to a 1-month-old," Goodrich says. "They have to learn to sit. They have to learn to roll. They have to learn to walk. They basically go through a yearlong period of a second infancy."

Nicole and Christian know well the milestones the twins have missed in their first year, like crawling and walking. They marked those with their oldest son, Aza, now 3.

Sitting on a park bench outside the hospital, Christian lays out his hopes and dreams. He longs to roughhouse with the boys and chase them. Nicole simply wants to pick them up when they cry and rock them to sleep.

The boys don`t have cribs. They share a queen-sized mattress with a pillow between them. There`s no stroller big enough to allow for walks around the block. The same goes for car seats: When the twins are transported, Mom and Dad must place them on the floor of their minivan and drive very carefully.

It`s those simple everyday moments of parenting they miss. Even the thought of pulling a T-shirt over the boys` heads can send Nicole into a fit of tears. That`s something she`s never been able to do. The boys only wear onesies.

Nicole McDonald says she and her conjoined twins, Anias and Jadon, are "in the final lap of this part of the journey."

The parents say they are telling their story to honor the medical staff for taking care of them over the last several months -- and they hope their message can help other parents struggling with a sick child. "To let them know these things that are difficult," Christian says, "can be a blessing in disguise."

Nicole, 31, worked as a pediatric physical therapist, and has been helping the boys with their motor skills, preparing them for being separated. Her favorite thing now is sitting quietly in the living room, listening to them babble back and forth in their bedroom, as if they`re talking to each other.

For Nicole and Christian, life nowadays is a collision of fear and joy, of excitement and anxiety. There are so many unknowns about prognosis and survival the McDonalds have had to find comfort in uncertainty. Christian says they understand "one or both twins can be neurologically challenged or they can have a mental or physical handicap" as a result of the operation.

"We know that is definitely a real possibility, but we`re still going to love our boys," the 37-year-old father says.

They worry most about Anias. He has suffered with breathing and feeding difficulties, and has had vision and heart issues. He is fed by a machine hooked up to his stomach. He had seizures at one point, but they`ve since been kept in check.

Standing in the boys` bedroom, the parents agree that despite their worries, surgery is in the boys` best interests. There have been other craniopagus twins, similar to the McDonalds, who have grown old joined together, learned to walk and even get married. It is by all accounts a challenging life. Not the life Nicole and Christian want for their boys.

"We`re so excited," says Nicole.

"These boys, they`re raring to go," Christian says. "They can`t stay like this any longer."

Glancing over at the twins, the mother tells them, "In just a couple weeks when you cry, I`ll get to hold you."

The boys squeal.

More about: