In a mixing bowl, dissolve gelatin in boiling water. Add cold water. Measure 2/3 cup of the gelatin. Chill remainder until slightly thickened. In another mixing bowl, beat measured gelatin until fluffy, thick, and doubled in volume. Chill until set. Spoon half of remaining gelatin into parfait glasses. Add layer of marshmallows. Spoon whipped gelatin over marshmallows in glasses. Top with clear gelatin. Chill until ready serve. Garnish with additional marshmallows.