Use the wedge of grapefruit to rub the rims of 6 margarita glasses with grapefruit juice. Dip the rims in the kosher salt to coat. Set aside as you prepare the margaritas.
Place the tequila, vodka, simple syrup, grapefruit juice, lime juice and ice in a bar blender and puree until smooth. Divide the margarita between 6 glasses and garnish with the grapefruit quarters.