Congratulations! What pieces will you play?
I think it’s nice for the other harpist that she plays another instrument, because then she can also play the pieces with one harp.
And yes, you should know the piece 100%. If possible, listen to recordings and play along with it. When I was in youth orchestra, I didn’t know the piece 100% and I was very embarrassed during rehearsals. Now I’m replacing in professional orchestras and I’m always preparing the piece very well, no matter how simple it is! In fact, the easier the piece, the more difficult the rehearsals, as one is intended to pay less attention to the easy piece!