Bloody Mary Bloody Mary! BLOODY MARY!
To me it sounds like the kid doing this really believes in it. But what I suspect has happened (to him or her) is that doing this very late at night causes your paranoia and fear to increase. Also, for some, the possibility of seeing illusions (out of the corner of your eye) increases.
Sitting by 2 mirrors, at the corner of your eye with a candle flickering will cause your mind to create images of people in the two mirrors – an attempt to make out what is actually your reflection. Due to the wee hours of the night your fear & illusions will be at their peak, so as to create an unpleasant experience.
I’d do it just to see how fucked up my mind wants to make things. But no, you’re not going to open up some kind of portal to “The Shadow Realm” LOLZ