Bring pineapple and cherry juice to a boil. Add gelatin and stir until dissolved. Stir in sugar. Chill about 30 minutes or until mixture mounds slightly when dropped from spoon. Whip milk and lemon juice until stiff. Pour on top of gelatin mixture. Beat in slowly with rotary beater or electric mixer on low speed. Fold in cherries. Pour into chilled pie shell. Chill several hours until firm.