Two-year-old conjoined twins have been successfully separated in what an Italian hospital said was a first-of-its-kind operation.   The twins – Ervina and Prefina – had been joined above the nape of their necks.   They were born with a condition the Ospedale Pediatrico Bambino Gesù in Rome described Tuesday as “one of the rarest and most complex forms of cranial and cerebral fusion,” Agence France-Presse reported.   However, surgeons succeeded in separating the girls and they are doing well. …