The mirror is in the most northern house in Aleroth (the empty house with a bunch of packages piled up, where you found the book).
You can leave the town before curing the two soldiers, you just can not complete the escort quest (or the healers will move on their own and the magic mirror quest will fail).
Aleroth screenshot (spoilers)
BTW, do not tell either healer that you have cured their patient until you have cured both of them.
