Synergy - 2 computers, 3 monitors. middle monitor for its own pc
Synergy can't do that, yet. We are planning to get this included in a Synergy 2.x release but aren't able to give an ETA as yet.
We have his on our Github: Independent grid-less screen positioning #4173
For now you'll have to use Synergy keeping one computers screens as one unit and another computers screens as another.