Well, when Luke leaves Dagobah, Ben seems pretty convinced that it's Luke who is the "last hope", even though he had knowledge of both of the twins. Why he thought that can only be attributed to the Force, I suppose. But when Yoda refers to Leia as the "other" hope, it always sounded to me like she was the backup plan. If Luke happened to be killed, then Leia could take over. Because, throughout the OT, no one, not even Yoda, ever really seems to be interested in actually training Leia in the Force. I kinda wish it had come up, though, since the "there is another" revelation really seems to be a letdown since Leia doesn't seem to live up to that cryptic reference in Return of the Jedi.