Yep, I agree. You know, I've thought of that prophecy and wonder if we've seen "the two that are one, the one that is all" and wonder if the "twins" of old were all either 'male' twins or 'female' twins, not brother/sister twins. Back to the old yin/yang principle -- opposites, bound together, transforming and balancing one another -- unity.
We got to see a bit of "the two that are one" when Josh leant Sophie his strength as they fought alongside Joan and Francis. It's early on in the series and so I don't interpret the prophecy as one to save/one to destroy. What we have so far is that the twins of old both fought against one another and that fight brought about the destruction of Danu Talis. Neither alone could save it, neither alone could destroy it...but united "the two are one" and have the power to work together and save the world.