Are they 'seized' together, or is there just so much crud on there that they are 'stuck' together? The second scenario was the case with mine. I cleaned mine off with a screwdriver and wire brush as best I could, then hit it with PB Blaster every day for a couple of weeks. Backed them out together as you suggested; but just a few turns and not all the way. With it all loosened a few turns, put a box-end wrench around the nut. Put a screwdriver in the slot at the top (to keep it from closing in) and then clamp around that top part with some vice grips (only the part without threads, you don't want to mess those up). Before turning anything, visually bookmark where the wrench and vice grips sit in relation to one another so you know how much things have moved; very minor adjustments make a big difference.

At that point you should be able to push on the wrench while resting the vice grips against the fender or something. Just move it enough to break things loose, then return it all to the position you visually roadmapped.

Take off the vice grips, adjust the screw 1/4-1/2 turn clockwise while holding the wrench so the nut stays put, then take off the wrench and tighten the screw back down all the way with the nut on it. If you did things correctly, the screw will now be in its original position plus 1/4-1/2 turn clockwise. Take it for a drive and see what you think. If it needs more adjustment, loosen the nut only, turn screw another 1/4-1/2 turn, then tight nut back down without allowing screw to turn.