Grate the zest of 2 limes into a small bowl. Cut the zested limes in half, and squeeze the juice into a measuring cup to get 1/2 cup. Combine lime zest, lime juice, sugar and water. Cover, and refrigerate for approximately 24 hours.
Before serving, stir in the tequila and Grand Marnier. Rub rim of 4 glasses with sliced lime, and dip into salt. Take another slice of lime, cut in half, slice under skin of lime to halfway point, and place on glass for garnish.
