In the top of a large double boiler set over barely simmering water, melt the chocolates together. Stir well and set aside to cool slightly. In a mixer fitted with a whisk attachment (or using a hand mixer), whip the cream until stiff. Keep refrigerated.
In a clean, dry bowl, whip the egg whites until soft peaks form. Add the sugar and continue whipping just until stiff and glossy, about 30 seconds more. Whisking hard, incorporate 1/3 of the egg whites into the chocolate until well blended. Fold in the next 1/3 of the egg whites. Fold in the remaining egg whites until just barely combined (the mixture will be streaky). Add the orange zest, orange juice concentrate, and whipped cream and fold until smooth.
Use a pastry bag fitted with a large plain or star tip to pipe the mousse into serving dishes, or simply spoon it into a large serving bowl. Cover lightly and refrigerate at least 1 hour or until ready to serve.
Recipe courtesy of Gale Gand, “Butter Sugar Flour Eggs” by Gale Gand, Rick Tramonto, Julia Moskin, Clarkson N. Potter Publishers, 1999