The fact that they are lying through their teeth in these interrogations makes the whole 'still working together' a highly probable scenario. The way they rigged the scene after Hart kills Ledoux, they could easily stage a fallout and be secretely working together all this time. That would awesome, I hope series gets taken in that direction or at least that they start working together again to finally solve the case, although I don't think it'll ever be solved per se but at least give it some sort of closure.
Fuck, last 3 episodes are going to be such a ride. I'm not sure if I'm ready.