I heard someone had one twin vaginally and the other by cesarean, I wondered why.?

Good question! 6-25% of twins require c-section as an emergency procedure to save the life of the second twin after the first twin has delivered vaginally. That is why many obstetricians shy away from attempting vaginal delivery in anything but singleton cephalic fetuses. In fact, the us rate of cesarean section for twins is >80% at this time. The second twin has a 4-fold greater risk of dying at vaginal birth!