Final answer:
The appropriate principal diagnosis would be Placenta previa with hemorrhage.
Step-by-step explanation:
The appropriate principal diagnosis for a patient with a placenta previa, delivery of twins, and a history of two previous cesarean sections would be Placenta previa with hemorrhage. Placenta previa is a condition where the placenta partially or completely covers the opening of the cervix. Hemorrhage refers to profuse vaginal bleeding. This combination of factors in the patient's history and the emergent C-section due to hemorrhage would make placenta previa with hemorrhage the appropriate principal diagnosis.