-Somewhere in Ikebukuro-
Shizuo was chasing Izaya through the city as usual, throwing random heavy objects at him like always. "Stay still, flea!" shouted Shizuo. "But Shizu-chan, if I stayed still you'd hit me~" Izaya giggled back maniacally. Tormenting his favorite monster was always such fun! Suddenly, two magical beams of light shot out of the sky and engulfed them.
-Hogwarts School of Witchcraft and Wizardry: Slytherin common room-
It was late at night, and most of the students and faculty at Hogwarts were asleep. Izaya tumbled forward gracefully, looking around once he regained his balance and noting his new surroundings. Shizuo seemed to be unconscious in a corner of the room. "Where-?" as the information broker attempted to figure out just where they were, two young boys walked into the room, looking to be around 16 years old. One had very light blond hair, and the other had dark hair and glasses. Izaya could also see a faint lightning-shaped scar on the dark-haired boy's forehead, although he wasn't about to question it. The two boys seemed to be holding hands. Izaya shook his head just as they noticed him, he should be trying to figure out where the hell this was rather than making stupid observations about random strangers. He realized they'd noticed him once their hands separated quickly to hide behind their respective backs, and the blond one started questioning him in English. "Who are you?" he asked, "Where did you come from?"
So yeah, I might make a sequel at some point after I write some City of Heroes fanfiction, because this really is a terrible place to end it, but I'm not really into writing a Durarara! / Harry Potter crossover right now.
And in case you were just totally oblivious to who the two HP boys were (I get like that sometimes too), it was Harry and Draco. I did try to make it as obvious as possible, rather than just being like "two random boys barged into the room and you have to guess who they're supposed to be!".