From my experience (which might be weird and special case) if you yourself break out one group via the back wall and have the boat ready, they will want to escape without waiting for the other group, so you have to break them out simultaneously (meaning inside a very narrow window). If you haven't readied the boat, they will try to escape via the cliff and get into trouble with the guards, same as if you just open the door. If you or Wulbren smash the back walls and an eye or the patrolling guard notice, they'll sound an alarm and you find yourself in a fight. It might also be that the guard standing below the cliff is close enough to notice the gnomes escaping through the wall, I am not entirely sure there. I am also not entirely sure if the eyes and guards recognise that a cell is empty. My attempts to free our friends while the guards are still alive have always been very chaotic.