Found a nice way to make this a one man job. Use a hack saw to cut a notch into a nut of the same size as the one on your bolt. From below put this nut onto the bolt and clamp it with vise grips. this will keep the bolt from spinning. you can then hold the bolt still, and twist the holding nut with another wrench. works nicely.
I believe what Rheaton is saying is that he cut all the way through one side of a nut. (Rather than a notch). He then threaded the nut onto the bottom of the bolt below the actual nut he was tightening and then grabbed it with the vice grips. The nut having been cut through on one side became a clamp on the threads of the bolt holding it firmly.
Damn good idea in theory.