P.
I agree that as long as you save both dresses, they will both be special.
I had a different situation. Everyone wanted to buy my first a very special and expensive dress and I saw no point. I borrowed the one my niece wore (and apparently her cousin too) and then my second wore it as well. So 4 girls from 3 families have worn this very simple, inexpensive dress and nobody can really claim it for their children.
You don't need a foo-foo dress for the second. I think if you get a nice dress, any color, then they each have a special dress to hold onto. And since it's not an official christening gown, neither should feel slighted later in life.