Use ThenByDescending:
var hold = MyList.OrderBy(x => x.StartDate)
                 .ThenByDescending(x => x.EndDate)
                 .ToList();
You can also use query syntax and say:
var hold = (from x in MyList
           orderby x.StartDate, x.EndDate descending
           select x).ToList();
ThenByDescending is an extension method on IOrderedEnumerable which is what is returned by OrderBy. See also the related method ThenBy.