Stir tequila, lemon juice, lime juice and syrup in a large pitcher until completely combined. Chill until ready to serve. For serving, stir ice, lemon and lime slices into pitcher. Pour over ice-filled rocks glasses and top with lemon and lime slices.
Do ahead: Cocktail can be made 8 hours ahead. Cover pitcher and chill. Add ice, lemon and lime slices just before serving.