I try to make things equal with my 2 kids, but life doesn't always cooperate. I just do the best I can for each child as their needs arise and as my circumstances allow. I don't worry too terribly much about if they perceive this as being "fair". Life is never "fair" and is a lesson they must learn. They know that I do the best I can for each of them at the time.
If I were in your shoes, I would ask your younger DD to compromise by holding off for a year, going to work and putting a certain percentage of her earnings aside for college expenses. Her willingness to do that should help to demonstrate how badly she wants this education and hopefully your husband will see it as a sign that he may have been mistaken about her ambition.