Stir together the orange and lime juice with the sugar in a medium saucepan. As soon as the sugar dissolves, remove the pan from heat and stir in the gelatin. Cut away the skin and pith from the oranges and cut into segments. In a rectangular terrine, pour 1/2-inch of the gelatin mixture. Refrigerate for 20 minutes. Place a layer of oranges over the hardened gelatin mixture. Refrigerate for 12 hours. To make the sauce combine all the ingredients in a medium saucepan and bring to a boil, stirring gently until the sugar dissolves. Boil hard until reduced by half. To serve place the terrine in a basin of hot water for several minutes, then unmold it onto a serving platter. Cut into slices. Serve with the cooled sauce. Garnish with lime zest.
