i would imagine they will do, or do the two differnt films at the same time, as the characters are not the same, and film companies often do a couple films at a time.
they cant leave it too long as the 4 books dont span a long period of time, so cant risk the cast ageing too much between the films especially the the cullens as they are meant to remain the same. (unlike harry potter for instance, where each book spanned a school year, so the cast ageing between each film made sense)
they are filming them quick to, they were only cast in the roles around jan/ feb 2008 and are now filming the 3rd in the series!