But unlike most siblings, they can't get away from each other - they were born attached at the chest and abdomen.
On Tuesday, a team of doctors and nurses at Stanford University's Lucile Packard Children's Hospital are planning a nine-hour procedure to give "silent Inah" and "talkative Icah" their independence.
"We want them to live normally," said Ginady Sabuco, the girls' mother. "When they argue, they can be alone. When they play, they can play together or apart. When they don't want to see each other, they won't have to."
Dr. Gary Hartman, the lead surgeon on the girls' case, said keeping the girls joined carries bigger risks for their health. If one conjoined twin dies, the other will die within hours. Muscular and skeletal deformities can also worsen with time.
Hartman, who has done five separation procedures at medical centers around the country, was upbeat about the girls' prospects. "Our expectation would be that we will have two healthy girls at the end of the operation," he said.
Angelica and Angelina are classified as thoraco-omphalopagus - joined at the chest and abdomen. Their livers, diaphragms, breast bones, chest and abdominal wall muscles are fused. They have separate hearts, brains, kidneys, stomachs and intestines.
The operation will involve cutting along the girls' skin and muscle and separating their diaphragms and livers. Hartman said severing the liver will be the riskiest part of the procedure because of potential blood loss.
Doctors will then cut any adhesions between the girls' bowels and snip any remaining skin. Separate reconstruction operations will follow.
Hartman said he doesn't expect the children will need intense physical therapy following surgery but they may experience regression with some milestones they've already hit. The girls can walk now despite their face-to-face orientation but may lose that ability, for example, for a few weeks to months while they recover.
They were expected to be in the hospital for two to three weeks. Angelina and Angelica were born in the Philippines and live in San Jose with their parents and 10-year-old brother.
"We expect they will have a little separation anxiety too," said Ruby Sabuco-Collins, the girls' aunt. "But we're just so proud of them now and so excited for them in the future."
The occurrence of conjoined twins is estimated to range from 1 in 50,000 births to 1 in 100,000 births worldwide, and the overall survival rate is approximately 25 percent, according to the hospital.
This is the second set of conjoined twins separated at the hospital. The last such procedure took place in November 2007.