User blog comment:OH&S/Toaru Majutsu no Index VS Durarara!! Special Collaboration/@comment-24.189.131.124-20150124060658/@comment-4419516-20150124160702

From the few spoilers from animesuki:

In the first part (by Kamachi) we know that apparently Touma pisses Shizuo off because he accidentally rips a button of his bartender uniform which leads to the vending machine throw and being thrown into a car. Shizuo comments Touma is as durable as Simon because of this. A giant airship appears because of the distortion brought by the crossover that threatens to crash into AC. Mikoto can't hack into it and there are innocent Tokiwadai girls inside so Touma asks for help to the Durarara cast. Shizuo demands Touma shows him his resolve which leads to the fight where Shizuo punches him 15 times and Touma manages to get a weak punch before falling unconscious.

Kadota basically says that from the moment Touma accepted the match he had already won and Celty notices Shizuo held back but even she would have taking all those punches. The Tokiwadai group, Kazakiri, Anri, Celty and Shizuo stop the airship.

The second part (by Narita) Izaya tricks Accel into fighting Shizuo with help from Loki (who disguises him as a member of the Board of Directors). Izaya convinces him he's a threat to the Sisters and LO. Shizuo only gets a headache from Accel's bloodflow reversal. When he tries throwing a vending machine Accel refelcts it only for him to punc it back so they do this back and forth until it breaks. Because Shizuo can't actually do shit against Accel he starts getting beat around. Celty sees this and gets mad, trapping Accel in a shadow sphere to suffocate him and calm him down. Accel uses his wings to rip himself free. He admits that Celty could hacve killed him if she wanted to because shadows don't have density and are more a phenomenon than a physical material.

The crossover is apparently caused by the Absurd (A Simple Series)