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.