Quentin apparently knew, without being told, who Judah Zachery was, and I have the impression that Desmond knew who the Head was when he bought it. Desmond's original idea was that the Head would be a great joke, but if Quentin, with his knowledge of and belief in the occult, had been presented with the Head of Judah Zachery, what would he have done?
I think that neither Desmond nor Q#1 knew much, or anything, about Judah to start with. After Ben died, Daniel talked to Desmond about the Bedford atrocities, or about later incidents with a disembodied head, or both. Desmond did research on this at the newspaper. Being under Judah's spell revealed it all, I suppose.
Then Desmond had his first relatively spell-free day, and rushed to First Quentin, and then, off-camera, Desmond filled Q in on it all. When we the viewers come back, Q knows all about Judah because of Desmond.
My concentration is not good now, so I may have misunderstood the point on which there was confusion.