The ovulations might occur at the same time, or a couple of days apart. In horses, it is rare that a fertilized egg splits to form identical twins. The reason is that a specific part of the embryo must be in contact with the floor of the uterus. If this portion of the embryo does not have the specific contact, the embryo doesn't receive enough nutrition and dies on its own. So, if one embryo sits on top of the other one, the one on top would be reabsorbed.
Spontaneous resorption is very rare when embryos occupy separate horns.
Embryos travel as part of their early life, and even if fertilized in the same horn, might separate on their own. Those that do, along with those that occupy separate horns, develop into twin fetuses that compete for nourishment. Explains Mats H.T. Troedsson, DVM, Dipl. If there are two instead of one, there will be less than half of the nutrition available for each fetus. As a result, abortion of both fetuses is likely to occur in late pregnancy, although the triggering mechanism isn't exactly known.