Your veterinarian may be able to do a transvaginal needle aspiration of one of the fetus placental fluid. But often if the fetuses are too close, when one is aborted, the other dies as well. Or you can opt to do nothing, and wait to see if the mare reduces the embryos on her own. Only 1 percent of twin pregnancies are carried to term. Know that the hope of two viable foals is dim and could be dangerous to the reproductive health of your mare.
Many mares have double ovulations.
Those that result in twin pregnancies usually resolve themselves, but it is wise to check. This season, my aunt and uncle and their veterinarian are monitoring their mare early to detect possible twins. They recently received confirmation that a single fetus is present and healthy. For more information, contact you local equine veterinarian.