I hear some people on this site putting double blades on, how is this done? is there something that keeps the blades locked in place? i want to try this but not sure i really get how you do it. never heard of it until i came to this site. do you just stack one blade on top of another on the blade bolt or is there something that goes between the blade to lock them at 90deg. how good would it work with new blades closest to the ground and older blades(rounded ends) on top?