Ah right, thanks! I thought I saw one of the leaders disappear when I went onto GS, which is why I thought they were separate. Seems I was mistaken, having a second look.
While it may make sense that way with the legacy of previous games etc, I think it would have been clearer had they put Rise & Fall and Gathering Storm instead of just Gathering Storm. Part of what confused me is that in other games (not Civilization) you can play just the one XP.