I believe that MIB and Jacob are both the same. One represents the Good side, one the bad(bad only on the surface). I'm saying bad only on the surface because I believe that the main characters need to be coerced into doing Bad so that they may learn from it in the long run(see my other blog). Only through redemption can one find true salvation. I also believe that Jacob realizes that MIB is his alternate personality(sorry couldn't find a better way to describe MIB) but that MIB does not and believes he is his own person. MIB already ate because Jacob had eaten. Jacob knew this and was just asking him to be humerous.