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

Okay, if u 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 of not they're Shizuo and Izaya's alter-egos, of 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 u know, of have any possible inkling of an idea, please answer....I'm so confused right now. = w =