Dissolve each box of Jello into its own individual container, using 1 cup of boiling water per flavor (no more than 1-inch deep.) Chill until solid, about 3 hours or overnight.

Once solid, cut each of the different flavors into small square cubes.

Using a 9 x 13 glass dish, gently mix the cubes together in an even layer (being very careful not to break or tear them.)

In a medium-sized bowl, dissolve 2 envelopes of unflavored gelatin in 2 cups of hot water. Let the hot mixture cool slightly and then whisk in the sweetened condensed milk.

Let the mixture cool to the touch (if it's too warm, it could melt the Jello cubes.) Then evenly pour the mixture over the cubes of Jello.

Chill in the refrigerator overnight.

Cut into squares and serve.