are examples of mixed constructions in which an element, in this case the second
as (
as bad as, as good as) has been omitted. It is common, especially in spoken English, but it is incorrect and should be avoided. The sentence
We're sure they can judge a novel just as well if not better than us (
London Review of Books, 1987) should be corrected to
just as well as, if not better than, us (note also the punctuation).