Investigating my own slave issue, I noticed the two slaves been stuck over one ore. I couldn't talk to any of them, move any, do anything, even hitting one of them didn't help. Killing one helped though, the other one got unstuck.
So this is my solution to the problem. Unless you find a script that fixes pathing, make private mines with 3 or 4 small seperate areas - 2 or 3 veins in the area. Each will contain 1 slave that will not get stuck with the other slave.
It's just my opinion though... I prefer one working slave than 2 blocked.
EDIT: It can be solved also by making large mine without possibility to interfere within working slaves - each ore must have at least 2 hexes for extraction. Or make a vein busy by one worker - a worker will choose a vein and mark it as unexisting to other slaves? Dunno. Because it looked like there was only 1 hex for distraction, and both slaves wanted to work over same vein, even being in two different places.