I recall reading somewhere (I believe it was Orson Scott Card's How to Write Science Fiction and Fantasy) that you should either stick with one genre or the other. The reason he gave is that if you're popular in one genre, readers of the other genre won't know who you are.
My answer to this would be to make your every book a bestseller and not rely on your reputation to sell your writing. But that's another matter. :)
Despite this, he does have a point. The fantasy crowd is different from the sci/fi crowd, and I expect publishers are more or less the same; they generally prefer to publish one or the other (if this assumption is wrong, please let me know).
So here's my question: If you intend to write both fantasy and sci/fi (or any two largely different genres), should you publish with the same publisher you've been using for your first genre (or try to), or should you use a different publisher that publishes in the new genre?