We usually use them FPs for the second park, but it depends on your hours. If you are getting there at the first rope drop and staying till lunch, then switching, I would use the FPS for the second park. If you are staying all afternoon and switching late in the evening, then I would use the FPs for the first park.
Also crowd calendars will influence me sometimes, I use the FPs for the park which will most likely be the busiest. For the most part though, I use the FPs for the park that I will be at from noon-4 pm.