Yes, if you're creating a domain and not DCPROMO'ing into an existing then, then DCPROMO the R2 box first (patch it up completely btw), then DCPROMO the regular 2003 box after. Shouldn't have any problems.
Obviously...if you plan on upgrading the 2nd box to R2 eventually, I'd do it now.
I'm Certifiable, not cert-ified.
It just means my answers are from experience, not a book.