1. Chill a metal, plastic or ceramic mold in the freezer. Spread a 1/2-inch-thick layer of softened lime sherbet on the inside edge of the mold and smooth with the back of a spoon; freeze until firm.
2. Fill mold with softened lemon sherbet and smooth the top. Cover with plastic wrap and freeze for 5 hrs or up to 3 days. To remove bombe, wrap a damp, warm cloth around the mold. (If its still stuck, insert a butter knife or spatula along one inside edge to unmold.) Garnish platter with mint leaves and zest. Makes 4 to 6 servings.