Dr. Jekyll and Mr. Norrell

In your experience which is better: two shorter fantasies that follow a similar time line, each complete in themselves (a sci-fi example would be Ender's Game and Ender's Shadow), or a great fat novel that included all the story lines at once?

I've read that publishers want to know they'll have more than one book out of an author, but from the stuff I see on the shelf lately it seems big books are quite popular too. (I ask, of course, because I'm working on a project that has reached the point when I have to choose which way I'm going).

I think that preference is going to vary per editor and house. More likely, the editor will prefer two books (more revenue), but in terms of what you write, you shouldn't base this decision on what a hypothetical editor wants. Do what will serve the story and your audience best.