Originally Posted by
elihu23
...... Would using two wrenches, one to hold the cone and one to hold the locknut work in practice?.... .
That's exactly how it's done, the cone is held in the adjusted position, and the locknut is tightened up against it to keep it there. It takes a bit of practice, because the pressure of the locknut tends to slightly tighten the adjustment, but is you go slowly trial and error you'll get the hang of it.
As I said earlier a keyed washer between the cone and locknut helps, but don't use a spring washer. If the axle isn't keyed, use plain washers, or a spacer if required for axle spacing, or no washer.