What if Jacob & MiB are 2 sides of the same person. Let's be truthful, we're not sure what they originally look like. We can assume what we saw in the 1800's were their original forms, but there is no way to truely tell. So maybe Jacob is recruiting just one person to replace them both.
Possibly a person who has had both mindsets before (free-will & destiny) who he can split into 2 personalities all together. Just a thought.