In two separate rectangular pans (9x13-inch is fine), sprinkle each box of Jell-O gelatin. Add 2 cups of boiling water to each pan. Stir until the gelatin dissolves. Put in refrigerator for at least 3 hours, until the liquid turns into jiggly Jell-O.
Cut each flavor into small blocks. Carefully mix the blocks in a 9x13-inch pan.
In a different bowl, sprinkle 2 pouches of unflavored gelatin into ½ cup of cold water. When the gelatin blooms, add 1½ cups boiling water. Whisk it together until it dissolves. Add the can of sweetened condensed milk. Stir and cool.
Pour the milk mixture over the Jell-O blocks and stick it in the refrigerator for at least 3 hours (or overnight). Then cut into squares and serve.