Joined: Apr 2012
From reading the first chapter and seeing the anime, I can determine that it starts with the Akane arc (Novel 5 or episode 5 of Durarara x2 Shou onward).
I am not sure if it skips Hollywood arc (Novel 4 or episode 1-4 of Durarara x2 Shou).
I have yet to read more till an official volume comes out, but considering how Hollywood bleeds into the later arcs, they might be saving Hollywood's backstory till they show up in late in the Akane arc and afterwards.
Official novels are being translated, so one day, I'll be able to compare properly.
Thanks for reading. Have a great day. :)