This happens fairly often toward the end of the year when a clubber has completed most of the sections except, perhaps, the ones where you are required to bring an offering for 4 weeks in a row (Sparks) or bring a friend, etc. We never want to hold a clubber back so we assign the next book (which becomes the primary book) but keep both books current so we don't forget the sections that still have to be completed in the old book
Plus, if you print off the detail sheets for "Current Handbook Status" (which we use in club every week for listeners to mark which sections are completed), you get a sheet for all books that are marked current.