Well I used to do two as well but did not like the added noise through the guides. When you think about it all you are doing is a nail knot. Most guys use just one nail knot to secure their leader to the fly line so this loop knot is not any worse. Plus you have the flyline pulled over on itself creating even more friction so it should actually be stronger than a nail knot. I have not caught anything really huge, steelhead up to 16 pounds and a 38" striper that was pushing 20lbs and the single held. I do know guys that have taken 25lb steelhead using this setup without any issues whatsoever.
Actually a couple weeks ago I probably had the striper of a lifetime on and my rod broke before the loop knot