Ironically, this puts both in grave danger, as the overload on the big one can be just as lethal as the deprivation to the other. Many, many ultrasounds are needed in twin gestations to make sure they're both growing at roughly the same rate. If twins are identical, there's a chance that they might share the same sac, instead of there being a dividing membrane between them. This makes cord entanglement a real possibility, putting them both in mortal danger. Once again, ultrasound comes to the rescue to show the membrane that would put that worry to rest.
Placental accidents are more likely, too.
Placenta previa, wherein the placenta lies over the opening of the womb, blocking the route out for a vaginal delivery, is more likely because there's more placenta there, and there's less space for it to occupy. Placental abruption, a premature tearing away of the placenta, is also more likely than in single pregnancies. This event could result in significant blood loss and danger of transfusion for the mother and mortal risk for the babies. Delivery poses dangers as well. The jumbling together of two babies makes for frequent abnormal positioning of one or both twins.