The song that both Sala and Ange mysteriously know is called "Towagatari", which means something like "Eternal Story". The version that Ange sings is subtitled "Hikari no Uta" = "Song of Light"; the version that Sala sings is subtitled "Kaze no Uta" = "Song of Wind".
The duet version that they sing in the final episode is 1.) glorious; and 2.) subtitled "El Ragna".
Who or what is "El Ragna"? The Cross Ange wikia claims that it means "The Goddess", but I'm not sure on what basis they make that claim ("El Ragna" doesn't mean anything in Japanese, at least). I don't recall the show telling us what "El Ragna" means; is there perhaps some side material that does tell us what's up?