Muddle 6 cucumber slices, 1/4 cup each cilantro and sugar and 2 lime wedges in a large cocktail shaker. Add 5 ounces silver tequila, 4 ounces triple sec and the juice of 2 limes. Fill with ice and shake well. Rub the rims of 2 rocks glasses with lime wedges and dip in coarse salt. Fill with ice; strain the margaritas into the glasses. Garnish with cilantro sprigs and cucumber slices.