Fetus measuring 1.5 inches at 8 weeks. 1 yolk sac visible. Could it possibly be twins and only half of one was visible making it appear to be large?

Unlikely. The fetus is much smaller than 1.5 inches at 8 weeks - although the entire fetal sac is much larger than the fetus itself. At 8 weeks there isn't much to see, but it's unlikely to be a twin pregnancy.