I doubt that anyone has a better answer than this. Steven Moffat (the writer) probably hasn't thought through every possible detail of the Angels, just the ones he needed for the stories he's written so far.
If he writes another story about the Weeping Angels and decides to include mirrors, he'll come up with more answers then.
Until he does, we don't know whether an image of an Angel _always_ becomes the Angel, or just _can_ become one. Or whether the original Angel and the new one that its image becomes are separate Angels or the same Angel in two places (or, for that matter, in just one place--maybe the Angel can jump around from image to image, inhabiting any unobserved image that it wants at any given time).
For example, if the mirror image automatically becomes another, separate Angel, the two of them would be looking at each other, which is just like the end of "Blink", so they'd both turn to stone.
But if the mirror image doesn't become an Angel, or it's just the same Angel, then at worst it's looking it itself, which they already do whenever they cover their eyes, so there's no problem.
It would become another angel, so it will look at the other angel, they will turn both into stone until you turn off the lights cos then they are able to move again.