The Double Fisherman is a very solid and strong knot. It’s a variation of an overhand with two wraps on two ropes going around each other. When the ropes are loaded, two fisherman knots butt against each other. This knot can be tied using ropes of dissimilar diameter and needs to be dressed (cross-over of the knots are on the same side) and set with stopper knots on each rope.
Secure two ropes together to gain more length; make prusik loops.
Set the two ends together, pointing the ends in opposite directions. Wrap one working end around both ropes twice. Pass the working end back through the loops and pull tight. Flip the knot 180 degrees and repeat the process. Make sure the sides of the knot sit against each other, finish by pulling both ropes tight to clinch the two knots against each other.