Lightly coat a 9X13-inch pan with nonstick cooking spray and set aside.
In a large bowl sprinkle the unflavored gelatin (4 packets) over the 1 cup of cold fruit juice. Let it stand for a minute or so. Pour the boiling fruit juice over the cold fruit juice mixture and stir until the gelatin is dissolved. Pour the liquid jello into the prepared pan and refrigerate on a flat surface (no tilting!) until firm, 2-4 hours.
Remove the firm gelatin from the refrigerator. Using a small heart-shaped cookie cutter, cut the jello into hearts. Cut the hearts as close together as you can to minimize scraps (my children were more than willing to help "clean up" the scraps). Carefully and gently remove the hearts to a cutting board or plate. Using a flat, thin spatula may help lift the hearts up and out of the jello. When all the hearts have been cut, wash and dry the pan. Coat it again with nonstick cooking spray.
Place all the small hearts onto the bottom of the clean pan, spacing them evenly. In a medium bowl, sprinkle the 2 envelopes gelatin (for the white jello) over the 1/2 cup cold water and let stand for 1-2 minutes. Pour the 1 1/2 cups boiling water over the gelatin and stir to dissolve. Stir in the sweetened condensed milk and stir until the mixture is cool (if it is warm at all, it will melt the red hearts, but don't chill it in the refrigerator or it will set up - just get it to cool room temperature). Carefully pour the white liquid into the pan with the hearts. Pour around the edges of the pan and pour slowly as the red hearts will be liable to want to float to different areas of the pan.
Refrigerate on a flat surface until set up, about 2-4 hours. Cut into squares and flip bottom-side up to serve (if there is a thin film of white over the hearts once you've flipped them, you can easily scrape it off with a fingertip).
Notes
Juice: I used a no-sugar added cranberry cocktail fruit juice. You have to understand a juice like that will be far less sweet than traditional jello but I actually preferred it less sweet and more fresh-tasting. Choose a flavor of juice you prefer because that's exactly what it will taste like (in other words it's not going to come out tasting like strawberry flavored jello just because the juice is red). Pan: I used a glass pan but from others who have tried this around the internet, I suppose if you use a nonstick aluminum-type pan, you could try flipping the entire pan over so the jello releases in one beautiful slab. My glass pan was having none of that (i.e. big mess) so try at your own risk. Step-by-Step: see the very simple step-by-step photos below the recipe for a quick visual.