Juice enough orange and lime until you have the correct amount. If it's pulpy, strain through a fine mesh strainer. Rub a wedge of lime around the rim of the glasses. Turn the glasses upside down and press the rim into the margarita salt. Set aside. In a Boston shaker, combine the tequila, Cointreau, lime juice, orange juice and simple syrup. Add ice cubes, secure the top on the shaker and shake until very cold. Fill the glasses with crushed ice and pour the margarita over the ice. Add a splash of water and garnish with slices of lime and orange. Serve.