Dual-line trimmers use two lines to cut down the weeds. Both lines wrap around a spool inside the head of the shaft. If you don't attach the line to the spool properly, the line inside the head can jam or clog and slip off the reel, which will prevent you from ejecting more line when you need it. If your dual-line trimmer jams frequently, you should consider dropping the diameter of your trimmer line one size.
Set the butt of the engine on the ground. Hold the shaft in the air. Twist the top of the head to release the cover. Pull the cover off the head.
Remove the inner reel and spring from the head of the trimmer. Cut or pull off any old trimmer line still on the reel. Clean the spool and head with a rag.
Measure out the appropriate length of trimmer line for your trimmer model. Double that number and cut it off the roll with the scissors.
Attach the first line to the upper half of the spool. Thread three-quarters of an inch through the eyelet on the spool. Press the line against the spool with your thumb.
Wind the line around the spool according to the arrow you see on the top of the inner reel. Wind the line in tight, even rows. Continue pulling the line taut as you wind it along. Rewind the line and start over if you lose tension on the line while winding it.
Insert the remaining 8 inches into the notch on the upper flange of the spool. Attach the second line to the lower half of the spool. Press the line down and wind it around the spool in the same direction. Insert the remaining 9 inches into the notch on the lower flange of the spool.
Press the spring and spool down into the head of the trimmer. Insert both line ends into the eyelets on the head cover. Secure the cover in place by turning it in the opposite direction.