Ok out of 4 times pre ordering and trading in at least 4 games each time, twice I was given both offers on my trade in preorders, twice I only got the preorder credit no extra 5$ for tradeins. I guess it will depend on the clerk. My advice if the clerk looks like a jerk dont try it, if hes a manager dont try it, but if they look cool just kinda ask nonchalant if both deals are combinable, two times I did get the deal I just asked hey I was wondering if those deals are combinable, I didnt demand it or anything. And the posters above are correct, they take your game trade ins and give you credit, at that point they can give you the extra 5$, then they will do the transaction with the preorders, both transactions should be independent of each other so offers CAN stack if the want to do it for you.
Oh and for Halo 2 orders, when I preorderd the guy told me they didnt have the special edition, when I went back later they had it so I asked if I could change it, the chick told me that the first 150 people that preorderd Halo 2 will get the special edition.