According to Udon's release schedule, the company plans on releasing "R20: Rockman & Rockman X Official Complete Works" in North America in two individual volumes, splitting the original 336 page book in half:
The first volume, Mega Man: Official Complete Works, will presumably contain all the classic series contents of R20 while the second volume, Mega Man X: Official Complete Works will represent the later, X series half of the book.
The books won't be released simultaneously; MMOCW is coming sometime September 2009 while MMXOCW will arrive in November. No additional information other than that, but this is great news to say the least.
Thanks for the tip Anon