ok, well, here's the logic... if The Incredible Hulk dvd is already in stores(say maybe a week or 2 before Iron Man dvd comes out), it might draw that initial craving for the General Audience's "superhero action flick", and serve as an appetizer before the release of the main course Iron Man.
plus, when people go to buy the Iron Man dvd, the Incredible Hulk dvd will already be in the stores, thus further tempting people to buy both DVDs if they haven't already got the TIH dvd yet. and well, with that said, i guess Marvel could release both dvds at the same time. but, i still think releasing TIH a week or 2 before IM is a better way to capitalize on that "appetizer" craving i talked about.
and the reason why i think it would not be wise to release the TIH dvd AFTER Iron Man is because most people will probably not care as much to come back just for the TIH dvd. whereas, like i stated above, it's better for Marvel to try to take advantage of the 2-in-1 day shot.