The agency said the operation required separating the intestine, liver and pelvic wall, and involved nine stages.
The twins were flown to Saudi Arabia in December for the treatment, after being transported from Gaza to Jordan, Saudi media say.
Facilities to treat their complex condition were not available in Gaza, where the UN says quality of health services has deteriorated due to a blockade, repeated cycles of armed conflict, Palestinian political divisions and budget cuts.
Israel and Egypt have maintained a land and sea blockade on Gaza for a decade in an attempt to prevent attacks by militants based there.