There's no reason why they shouldn't take your entree back while you wait for the other. I'm really uncomfortable eating while someone else just sits there. I'd rather enjoy the meal together. And if it's at a nicer place -- well, they just shouldn't bat an eye at that. If anything, they should be mortified that anything went wrong in the first place and make sure to take care of it however you'd prefer. Asking them to keep your entree warm isn't an egregious request.