1 cup white sugar, or more to taste
10 eggs, divided
1 quart milk
1 quart heavy whipping cream
1/4 cup rum
1/4 cup whisky
Beat sugar and egg yolks together in a bowl using an electric mixer until very light in color. Mix milk and cream into yolk mixture.
Beat egg whites in a separate bowl using an electric mixer until stiff peaks form, adding more sugar, if desired. Fold egg whites into milk mixture until eggnog is smooth. Stir rum and whisky into eggnog. Refrigerate until chilled, at least 1 hour.