Using 1 cup of boiling water per box of Jello, dissolve each flavor separately, whisking until completely dissolved. Pour each flavor into individual containers and chill for at least 4 hours or overnight.

Once the Jello is set, cut each flavor into small cubes and place them together in a glass 9×13-inch pan. Carefully mix the Jello cubes.

In a separate bowl, dissolve both envelopes of unflavored gelatin in 2 cups of hot water. Let cool slightly, and stir in the sweetened condensed milk. Cool to the touch, and pour the cooled milk mixture over the Jello cubes. Lightly tap the pan on the counter to remove any air bubbles, then chill overnight.

To serve, cut into squares.