This knot is used to join two strands of greatly unequal diameter together.
To Tie a Double Surgeon's Knot:
Place the two lines parallel, with ends pointing in opposite directions. Using the two lines as a single strand, make a simple overhand knot, pulling the two strands all the way through the loop, and then make another overhand knot.
Holding both strands at each end, pull the knot tight and clip off the ends.