The operation for Eva and Erika Sandoval of Antelope began at 6am and was expected to last between 18 and 24 hours.
The rare operation will involve 50 hospital staff, and 3-D printed models of the 2 year old girl’s fused pelvis and blood vessels.
Doctors say there is a 30 percent that one or both of the twins will not survive.
The biggest threat to their survival is blood loss.
The girls have separate brains and hearts but they share a liver, bladder, and some of their digestive system.
Even though the twins are thriving together doctors say they surgery is needed for their long term health.