This animation should help. Spelunkers, rock climbers, and rescue crews often tie triple, quadruple (and I've even seen quintuple) fisherman's knots for extra safety. After studying the animation, you should easily be able to tie these safer versions (though I can't imagine a double fisherman's knot ever spilling). I even have one book of rescue knots where they recommend tying a quadruple fisherman's knot AND THEN taking the little ends that stick out of the knot and tying them in double overhand knots around the standing part of the rope, lol.