Put ice, tequila, simple syrup, Limoncello, and lemon juice in a cocktail shaker or mason jar.
Cover the container and shake to combine and chill the ingredients.
Coat the rims:
Spread salt in a shallow dish wide enough for the rims of your glasses. Cut a slit in the lemon wedge and rub the lemon around the rims of the glasses to wet them.
Dip the rims of two margarita glasses into the salt.
Pour and serve:
Carefully add ice cubes to the prepared glasses. Pour the margarita mixture over the ice, being careful not to knock salt off the rims. Slip a lemon slice into each glass.
Serve and enjoy the drinks immediately!
Notes
Salt on the rims is up to you, of course. Feel free to omit it, or even try some fine sugar on the rims instead.