The Ivys and liberal arts places I know take multiple children going to school at the same time well into account. I'm sorry for the trouble, but I'm not sure if you didn't look into it, or if you are talking about a situation well into the past. That's just not the case anymore.

EDIT: Hell, look at the comment below; the net price calculator (http://npc.fas.harvard.edu/) even has a drop-down for number of children in college.