Emma is somewhere between shocked and impressed. She's arguing with Ollivander? It's Ollivander. She's not sure why you'd want a second wand- everyone only has one, right? That's how it works?- but that doesn't mean she understands why Ollivander is refusing.

"Why two?" she asks without thinking. Her mother elbows her sharply; breaking into other people's conversations isn't polite, Emma repeats mentally with a sigh. She mumbles, "Sorry t'interrupt," though she doesn't really mean it. She can't very well unsay the question, and she does want to know.