It'd be quite weird for me to hear Namie with a much "younger" voice and having extreme outfits (think "WestEast" magazine), while Ayu would be dancing in her PVs and showing some "style"
! But it'd be interesting to wonder about that, since most of them have certain parts where they excel at (like Ayu's many singles/albums getting released somewhat regularly), and parts where they may falter (like Ayu's dance moves), but at least hearing Namie sing the english part of Forgiveness would be somewhat easier to understand ^^