Line four loaf pans or other small pans with plastic wrap. In four separate bowls, dissolve one box of Jello in 1 cup of boiling water. Stir for 2 full minutes, until it is completely dissolved, then allow to cool to room temperature. Pour each into the prepared pans and chill at least 3 hours, or overnight.
Turn the chilled Jello out onto a cutting board and slice into ½-inch blocks.
Add the blocks to a jelly roll pan or 9x13-inch pan that has been lined with plastic wrap. Gently spread the blocks so they are in an even layer and the colors are evenly mixed.
In a separate large bowl, sprinkle 2 envelopes of unflavored gelatin into ½ cup cold water and allow to sit for 5 minutes. After the gelatin blooms (it will look mostly dissolved and get wrinkly on the surface), add 1½ cups boiling water and stir to dissolve. Add the can of condensed milk and stir to combine; set aside to cool to room temperature. Pour the cooled milk mixture over the red and blue Jello blocks in the pan. Chill overnight, or at least 3 hours until firm.
Cut into blocks or shapes and serve.
I lined my pans with plastic wrap so that I could easily lift the Jello out of the pan and turn it onto a cutting board to slice; you could spray your pans with non-stick cooking spray if you'd prefer.
Use any size pan you would like for the red and blue colors, but I've found the smaller, the better. If your pan is too big (I've done 8-inch squares in the past), the blocks come out looking pretty flat.
I have found that using a jelly roll pan is the perfect thickness if you're planning to cut shapes. If you are simply cutting into blocks, a 9x13-inch pan will give you thicker blocks and would work just fine.