Slide the two spools of Pearsall's onto the dowel. Be sure that the silk strands are coming off each spool in the same direction. Mount the combined spools in a standard bobbin holder and then thread both strands of silk thru the bobbin spindle. Grasp both strands of silk treat it as a single strand. As you wrap the fly, the thread will naturally form the segmentation of the body. Every eight to ten wraps, you will need to spin the bobbin to remove the twist. Continue to tye the fly as you normally would.
Since there are two strands of silk, it is very easy to split the strands apart to insert dubbing between the strands (no dubbing loop required). Just insert a small tuft of dubbing between the strands and spin the bobbin to lock it in place. Wind the dubbing onto the hook to finish the thorax.
The head on this fly is a bit big, because it was tyed with the dual strands of silk. If you want a smaller head, just clip off one of the strands before whip finishing the head. You choose which color the head will be, by clipping off the other color.
Close-Up view of the segmented body:
Mix and Match the colors of silk to form different bodies. If figure there is around 552 different color combinations to play with. Seems like a useful little technique. Give it a try and let me know what you think.