Single Worm Screws - work best when trying to clear a more solidified type of obstruction such as rags, fats, grease, silt or a severe root mass as these worms can be repeatedly wound into the obstruction like a cork screw to either break up the blockage and let it wash away or retrieve it for disposal.
Double Worm Screws - work best when the blockage is perhaps of a larger or looser nature as the twin prongs can be wound into the obstruction until they grip hold which enables the blockage of perhaps rags or roots to be withdrawn and disposed of.