Whom is Shiki talking to in the dream where he meets a serial killer (in the Kohaku route)?
Tohno Shiki (who is really Nanaya Shiki) is meeting with Tohno Shiki (who is really Tohno Shiki) in that scene. They don't recognize each other.


  SHIKI catches a drugged Shiki going berserk and killing some people in a back alley. Both have coffee on a park bench while discussing life and how they both fit into the world, and joking with each other like friends. SHIKI even points that Shiki reminds him of his precious childhood friend, not realizing that he is the same person. In the end, SHIKI does try to start a fight, but Shiki tells him that he's superior in a fight to the death. Surprisingly, he agrees with him, and decides to give up and leave town for good. 


Source: http://typemoon.wikia.com/wiki/SHIKI_Tohno#Kohaku.27s_route