Prepare the simple syrup. In a small saucepan, bring sugar and water to a boil. Stir constantly until sugar dissolves. Reduce heat, and simmer for 5 to 7 minutes. Remove from heat and cool completely.
Mix sugar and salt and place in a shallow plate. Run a lime wedge around the rim of two glasses. Dip the glasses into the sugar-salt mixture. Shake excess. Set aside.
Add tequila, lime juice, simple syrup, orange liqueur, and ice into a cocktail shaker. Shake for 15 seconds.
Pour mixture into the rimmed glasses. Fill with ice. Garnish with lime slices.