We've had lots of twins over the years. One year when we were about half done calving, we had about a 115-120% (can't remember exactly) calf crop!! A few things that I noticed....
A) Females that have twins almost always need to be cleaned a few days after calving.
B)Some females will mother both twins & let both nurse. Others will pick one & go off & forget about the other.
C) If the female lets both nurse, you almost have to supplement her nutrition or she will lose her condition fast!
D) I don't recall any arriving too much earlier than the due date, although they could.
If it were me & you KNOW that you are going to have twins, I'd put the female in a small lot by herself where you can watch her closely & you can provide whatever assistance is needed. Best case scenario you will have to clean her & feed her really good. Worst case scenario, you will have to force her to let both nurse, or if she wont accept one or if doesn't have enough milk, you may have to bottle feed one.