There are several youtube videos of these two. Jonathan is incredible. However, I was surprised to listen to other songs by these two and learn that Charlotte has a great voice as well. With the same microphone volume (for this appearance) there is no way anyone’s voice can compete with Jonathan.
Watch some others and tell me what you think. The right microphone level makes all the difference in the world and while very few are as good as Jonathan, I think you will appreciate her for her voice as much as her heart.