Threaded rods are used to move the axes from side to side. One down the middle of your machine is all you need. Your stepper turns the rod and the movable carriage has a nut that helps it move. The advantage to two rods is to eliminate the dresser drawer effect, it keeps your axis moving straight instead of rocking as it goes. There are other systems such as belt drive, rack and pinion, ect. Each has its advantages but using a trreaded rod is probably the easiest.