CHOP said the seven-hour procedures to separate Allison and Amelia Tucker of Adams, N.Y., were completed at about 3:40 p.m. Wednesday.
The twin girls shared their chest wall, diaphragm, pericardium and liver, Children's Hospital of Philadelphia said.
Lead surgeon Holly Hedrick said in a statement Wednesday night that the complex surgery "went very well and as expected."
Hedrick said doctors expect that after the surgery and a period of recovery and therapy, the girls "will be able to live full, healthy and independent lives."
Their mother Shellie Tucker started a blog when they first learned the babies were conjoined.
She wrote, "My husband and I were in complete shock (we still are). "
She went on to say, they "didn't have a clue what this meant."
Then they came to the Children's Hospital of Philadelphia. She had a C-section at 35 weeks. That was in March.
Recent pictures posted show the girls now, still together, with their parents, and big brother Owen.
Doctors here have been planning separation surgery for months. The lead surgeon says she expects after the operation, "recovery and therapy, the girls will be able to live full, healthy and independent lives."
Yesterday, 14 hours before surgery, Shellie wrote on CaringBridge.org:"We have signed our papers we have met the surgeons. We have done all a parent can do. At this point it's in the surgeons hands. We just have to hold our breath and hope and pray God listens to our 1000s of supporters. We will be okay. We will be strong."
Officials said the surgery will be the 21st separation of conjoined twins performed at the hospital.
Conjoined twins occur once in every 50,000 to 60,000 births, and most are stillborn. About three-quarters of such twins are female and are joined at least partially in the chest, sharing organs. Chances of successful surgery and survival are greater if the twins have separate sets of organs, the hospital said.
A year ago, conjoined 20-month-old twins from the Dominican Republic were separated at a Virginia hospital. Maria and Teresa Tapia were attached at the lower chest, and surgical teams successfully divided their liver, pancreas and other shared organ systems and reconstructed their abdominal walls.
A few days before that, 2-year-old twin sisters Angelica and Angelina Sabuco from San Jose, Calif., who were joined at the chest and abdomen, were separated in procedures performed at a California hospital.
These are some of the first pictures taken via ultrasound last year of Allison and Amelia Tucker. The twins are joined at the lower chest and abdomen. They share a liver, diaphragm and the sac around their individual hearts.