Imagine mapping my brain into two interpenetrating networks. For each brain cell, half of it goes to one map and half to the other. For each connection between cells, half of each connection goes to one map and half to the other.

What would happen in this case is that there would be no Manfreds, because (even assuming the physical integrity of the neuron-halves was preserved) you can't activate a voltage-gated ion channel with half the potential you had before. You can't reason about the implications of the physical reality of brains while ignoring the physical reality of brains.

Or are you asserting no physical changes to the system, and just defining each neuron to be multiple entities? For the same reason I think the p-zombies argument is incoherent, I'm quite comfortable not assigning any moral weight to epiphenomenal 'people'.