Mix unflavored gelatin and sugar in a medium microwaveable bowl until blended. Stir in soda. Let stand 3 minutes or until gelatin is softened. Microwave on high 3 minutes or until gelatin and sugar are dissolved, stirring after each minutes. Pour into 8-inch square dish sprayed with cooking spray.
Refrigerate 25-30 minutes or until set but not firm. Meanwhile add boiling water to flavored gelatin mixes in separate medium bowl; stir 2 minutes until completely dissolved. Stir in cold water. Cool 15 minutes.
Pour flavored gelatin gently over clear gelatin layer in dish. Refrigerate 1 hour or until firm. Cut into squares.
Recipe by Five Little Chefs at https://www.fivelittlechefs.com/recipe/desserts/valentine-jello-hearts.html