Can someone explain this "Tsugaru and Psyche" business to me please?

Okay, if you don't know, they're from Durarara and essentially, they're people who are identical to Shizuo and Izaya, except in different clothing.

But it's confusing me, because I can't tell whether or not they're Shizuo and Izaya's alter-egos, or just inhuman versions of themselves, like clones, designed just for Izaya's mere amusement.

I haven't read the novels, so is that where they're from? And I've heard it has something to do with the character art for Psycedelic Dreams, but I didn't understand that much...

...so if you know, or have any possible inkling of an idea, please answer....I'm so confused right now. = w =