Am I right the photo is not showing the state you are describing, since I don't think I can see the "round nut with two flats on"?
If you were able to loosen the top locknut there should only be limited traction between any other nut below and the hollow axle, i.e. you should be able to use a pair of pliers or even your fingers to loosen the next nut while holding on to the locknut on the other end. The threading you can see on the freehub is for tightening the cassette lock ring, ignore that.
If that is not possible for whatever reason, and if a standard 6 sided locknut and a nut with two flats are exposed on the other side, you can use a cone spanner (for the two flats) and a normal spanner (for the 6 sided nut) to loosen the locknut that side. Once the locknut is loosen the next nut should loosen easily.
Assuming this is the same hub/wheel you were talking about in the
other thread, are you sure this is not a standard cup and cone bearing hub (as opposed to cassette sealed bearing hub)? Can't be 100% sure yet, but that's what it looks like to me. Park has a good
guide on how to service cup and cone hub, for which you will need two suitably sized cone spanners to set up correctly.