There is a single nut on my 4" precision table. The nut has a cut in from the side so that it's allmost cut in half. There is a screw going through both parts so that when you tighten down the screw, the two "halves" are pressed together and so keeps the nut from rotating. I don't know if this is something you can do on your rotary table.
You can also drill and tap a hole in one of the nuts and put a set screw through it. This way you can tighten down the set screw into the other nut to keep it from rotating, and also putting more pressure on the threads to keep the nut itself from rotating. Don't know if this makes any sense.