I got this idea yesterday, and it became this theory. I don't think that the prophecy refers to the twins or
to the swords. I think it's the five immortals on Danu Talis.
I'm going to assume (for this theory) that the immortals cannot die for specific time-thread related reasons. In which case there is, technically, two of each of them--one which will be in history, and them, on Danu Talis. But, as they are the same people, they are, quite literally,The two that are one, that one that is all.
The phrase One to save the world, and one to destroy it
is not stated in the prophecy. That's something that people like Flamel, who have translated the prophecy, have said, assuming that the two are twins.
And that's my crazy crazy idea.