They do have to be separate orders since the half off only applies to used games that you buy with a preorder. It's in the title.
And I got Madden 11, RDR, Crackdown 2, Transformers WFC, Lego Harry Potter all for around $100 and then I cancelled my preorders. I wouldn't have been too upset if it wouldn't let me cancel, but it did so awesome deal, imo.