Just because they are twin heifers does not mean they are sterile. You may have heard of the situation where a heifer is born twin to a bull. Those are called freemartins, and about 95 percent of those heifers are incapable of reproducing. Most scientists believe that the cause of freemartins is that the male reproductive tract (of the embryo) develops earlier than the female. The early presence of the male reproductive hormones causes underdevelopment (if not complete impairment) of the reproductive tract in the female twin embryo, therefore resulting in a female calf that looks normal but has a very underdeveloped and nonfunctional reproductive tract.
In the case that you described, the twin heifers (if a male embryo was not in the uterus at any time during gestation) should be normal and should be able to reproduce normally.
