My daughter is looking for a young adult, or maybe kids, book that she read about a normal girl who one day notices a boy looking back at her through a mirror. Then he pulls her through into another world where they have dragons as familiars, there are talking cats and everyone has a corresponding person/soulmate/double of some kind.
It's supposed to be part 1 of a series or just has a sequel. It ends in a cliffhanger, and she can't figure out what the book was to finally read the sequel. She doesn't remember any character names.