Stir the bittersweet chocolate and butter in large metal bowl set over saucepan of simmering water until smooth. Remove from the heat. Stir in the desert wine or liqueur. Cool to room temperature, whisking occasionally, about 30 minutes.
Beat 1 cup cream with 2 tablespoons sugar in medium bowl to soft peaks. Fold into the chocolate mixture. Divide the mousse among 6 wineglasses or soufflé cups. Decorate each mousse with a slice of orange. Cover and refrigerate until set, about 2 hours.
Combine the white chocolate and 2 tablespoons cream in medium metal bowl set over a saucepan of barely simmering water until melted and smooth. Remove the bowl from the heat. Cool the chocolate to room temperature.
Beat the remaining 6 tablespoons cream in another medium bowl to soft peaks. Fold the cream into the white chocolate mixture. Refrigerate the mixture until firm, about 1 hour. Pipe or dollop the white chocolate cream atop center of each mousse.