To a cocktail shaker or cocktail mixing glass add the tequila, mezcal, cointreau, lime juice, and simple (if using). Top with ice and stir/shake. Taste for simple adjustment. For the rim, mix the sea salt and cane sugar together in a small shallow dish. Rim a low-ball glass with lime, then dip into salt/sugar mixture. Fill the glass with ice and strain the cocktail mixture into the glass. Top with club soda and garnish with lime wheel.
*Homemade Simple Syrup: Bring equal parts sugar and water to a boil (1/4 C each) and simmer until all the sugar is dissolved. Pour into a storage container and refrigerate until ready to use.