Until just recently it was thought that all fraternal or dizygotic twins must have separate placentas, since they come from separate eggs. However, this week in the New England Journal of Medicine it was reported that a through investigation of fraternal twins born with what was believed to be a single placenta was indeed accurate. It is extremely rare for this to happen, and there is only one other report in medical literature. It is believed that the most recent case is due to the use of in vitro fertilization.
I wonder if it could have also been a case of fused placentas or something like this section: Twin Placentas