Nine-month-old twin girls Allison June and Amelia Lee Tucker were joined at the chest and abdomen, sharing a chest wall, diaphragm, pericardium and liver.
But, because they didn't share a lot of internal organs, doctors at the Children's Hospital of Philadelphia are confident of the girls' prognosis.
Pediatric surgeon Dr Holly Hedrick says Allison is the smaller but tougher twin, and Amelia is the larger twin, who is still in intensive care recovering from the operation.
The complex surgery - which involved 40 medical staff - is the 21 separation of conjoined twins performed at the hospital.
The twins' mother, Shellie Tucker, who is married to their dad, Greg wrote on her blog:
Caters
"Today it is a time to celebrate, to be thankful, to appreciate, to praise, and to enjoy this weight lifted off our shoulders.
"The surgery went better than expected. The girls did amazing. The surgeons were able to separate the girls and close them with the skin they grew over the last five months. After the girls left us yesterday I had a sense of calm and strength come over me. These girls are going to do big things in life!'
Speaking to reporters after the operation, she added: "Greg and I are thrilled that our daughters have been given this remarkable gift by the amazing team from The Children's Hospital of Philadelphia.
"We are so grateful for the support and encouragement that the medical team, our families and our friends have given us during this long journey."
Huge congratulations to all the medical staff involved - and a very merry Christmas to Shellie, Greg, Allison and Amelia from everyone at Parentdish.
Conjoined twin boys sharing three legs separated in 24 hour operation