The coarse threads on those studs are pretty short as I recall. Might not be enough room to get two nuts on there.
If that is the problem, take a single nut and cut through one side of it. A Dremel tool with abrasive cutoff blade makes short work of this, but it can be done with a hacksaw and patience.
Place the split nut on the threads, then clamp it there with vise grips or your bench vise.
After removing the other nut, and releasing the pressure from the split nut, it should come off easily.