In a medium-large, non-reactive saucepan over medium heat, reduce orange juice to 2 cups. Add sugar and dissolved arrowroot to reduced orange juice. Simmer for 1 minute, until mixture is very slightly thickened.
Cool. Add lemon juice and lemon & orange zests. Cover and refrigerate until needed. Sauce will be thin. Sauce keeps, refrigerated, for at least 3 weeks.
To serve, spoon onto plate next to dessert. Decorate with candied orange zests, if desired.