I like the main mechanic, I've seen a few games that have player input controlling multiple characters, but I this is the first one I've seen so far where the other controller character is the antagonist. One problem I have is that it's a little too easy to lose the enemy character. Especially in the early levels, it's real easy to just get him to walk off the ledge, and you can then walk to the end of the level with little trouble. Maybe multiple doppelgangers that spawn at different start locations would help with the difficulty? I think there's a lot you could do with this mechanic, and it's a shame about the time limitations of a game jam, as I would like to see more.
I also really liked the design of the player character and especially the bad guy, and the effect where he is first reduced is awesome.