Well full term is 38 plus weeks anywhere from 38-40 weeks. That is no different for twins however twins usually come before 40 weeks. This does not mean it isn't possible to make it to 40 weeks bc women do. I think typical is 38 weeks with twins.
my ob told me 38 week is pretty much full term with twins but most women don't make it that long. I had my guys at 34 weeks.