I would do whatever makes the most sense in a given situation. For instance, with my kids jumping around, I find it easier to preview books than to PhotoRead. I would preview/prepare several together. Since I'll be defining the purpose for PhotoReading each book before I start PhotoReading, there shouldn't be any interference or confusion.
As far as the sequence for dealing with separate books, I don't see how it would make a difference. Each book needs its chance to be processed through the inner mind. This should be irrespective of what you're doing with another book. Go with whatever feels right.