CloisteredFlame
Yuki is my reason for existence
I really think it's just Joelle doing it live. It's definitely not outside of her head voice range and the vocals for this song were not so studio-stacked (like kajiurago in some FJ songs or lots of Kalafina tracks) to require Yuriko's backing vocal tracks for a fullness effect or because it's too high for those on stage.At the very very end, tell me that is not Yuriko's voice popping out when Reona, Asca, Kaori, and Joelle stop singing. It doesn't sound like any of the other girls to me. In this video it happens at 6:50:
https://www.youtube.com/watch?v=jsoRBfl-WWE
ASCA was singing the low harmony, ReoNA the middle harmony, Kaori the melody part, and Joelle the high harmony (higher octave to ASCA's part). I think it was just because Joelle sang it with a much more "hooty", classical tone (which she's capable of doing) that it sounded like Yuriko (who tends to get to do that a lot in her role up top).
I think it's sort of like how classical sopranos can often sound so alike singing the same parts because they're using the exact same techniques.