Wash the limes in hot water and grate them dry. Finely grate the zest of 1 lime. Finely grind the salt and lime zest in a mortar. Mix grapefruit juice, orange liqueur, tequila and sugar
Wash grapefruits hot and rub dry. Cut grapefruits and limes into thin slices. Spread the grapefruit and lime slices on the glasses, fill up with ice and pour the tequilla mixture on top. Sprinkle with lime salt