• Never did the 9s on an 11s freehub but I reckon you’d need both spacers if the casette is still slightly wiggly when it’s tightened down with the 1.8mm spacer on. The 1mm spacer helped me to tighten wobbly casettes when a normal casette spacer would have pushed the smallest sprocket too far off the freehub. In case you have a parts bin with old casettes: you can find the 1mm spacer on some casettes between the two smallest sprockets: